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Some signs are specifically characteristic and indicative of a disease and are called pathognomonic signs; but these are rare. Not all infections are symptomatic. In children the presence of cyanosis , rapid breathing, poor peripheral perfusion, or a petechial rash increases the risk of a serious infection by greater than 5 fold. Microbiological culture is a principal tool used to diagnose infectious disease.
In a microbial culture, a growth medium is provided for a specific agent. A sample taken from potentially diseased tissue or fluid is then tested for the presence of an infectious agent able to grow within that medium. Most pathogenic bacteria are easily grown on nutrient agar , a form of solid medium that supplies carbohydrates and proteins necessary for growth of a bacterium , along with copious amounts of water.
A single bacterium will grow into a visible mound on the surface of the plate called a colony , which may be separated from other colonies or melded together into a "lawn". The size, color, shape and form of a colony is characteristic of the bacterial species, its specific genetic makeup its strain , and the environment that supports its growth.
Other ingredients are often added to the plate to aid in identification. Plates may contain substances that permit the growth of some bacteria and not others, or that change color in response to certain bacteria and not others. Bacteriological plates such as these are commonly used in the clinical identification of infectious bacterium. Microbial culture may also be used in the identification of viruses: In the case of viral identification, a region of dead cells results from viral growth, and is called a "plaque".
Eukaryotic parasites may also be grown in culture as a means of identifying a particular agent. In the absence of suitable plate culture techniques, some microbes require culture within live animals. Bacteria such as Mycobacterium leprae and Treponema pallidum can be grown in animals, although serological and microscopic techniques make the use of live animals unnecessary.
Viruses are also usually identified using alternatives to growth in culture or animals. Some viruses may be grown in embryonated eggs. Another useful identification method is Xenodiagnosis, or the use of a vector to support the growth of an infectious agent. Chagas disease is the most significant example, because it is difficult to directly demonstrate the presence of the causative agent, Trypanosoma cruzi in a patient, which therefore makes it difficult to definitively make a diagnosis.
In this case, xenodiagnosis involves the use of the vector of the Chagas agent T. The bug is later inspected for growth of T. Another principal tool in the diagnosis of infectious disease is microscopy. Virtually all of the culture techniques discussed above rely, at some point, on microscopic examination for definitive identification of the infectious agent.
Microscopy may be carried out with simple instruments, such as the compound light microscope , or with instruments as complex as an electron microscope. Samples obtained from patients may be viewed directly under the light microscope, and can often rapidly lead to identification. Microscopy is often also used in conjunction with biochemical staining techniques, and can be made exquisitely specific when used in combination with antibody based techniques. For example, the use of antibodies made artificially fluorescent fluorescently labeled antibodies can be directed to bind to and identify a specific antigens present on a pathogen.
A fluorescence microscope is then used to detect fluorescently labeled antibodies bound to internalized antigens within clinical samples or cultured cells. This technique is especially useful in the diagnosis of viral diseases, where the light microscope is incapable of identifying a virus directly. Other microscopic procedures may also aid in identifying infectious agents. Almost all cells readily stain with a number of basic dyes due to the electrostatic attraction between negatively charged cellular molecules and the positive charge on the dye.
A cell is normally transparent under a microscope, and using a stain increases the contrast of a cell with its background. Staining a cell with a dye such as Giemsa stain or crystal violet allows a microscopist to describe its size, shape, internal and external components and its associations with other cells.
The response of bacteria to different staining procedures is used in the taxonomic classification of microbes as well. Two methods, the Gram stain and the acid-fast stain, are the standard approaches used to classify bacteria and to diagnosis of disease. The Gram stain identifies the bacterial groups Firmicutes and Actinobacteria , both of which contain many significant human pathogens. The acid-fast staining procedure identifies the Actinobacterial genera Mycobacterium and Nocardia.
Biochemical tests used in the identification of infectious agents include the detection of metabolic or enzymatic products characteristic of a particular infectious agent. Since bacteria ferment carbohydrates in patterns characteristic of their genus and species , the detection of fermentation products is commonly used in bacterial identification. Acids , alcohols and gases are usually detected in these tests when bacteria are grown in selective liquid or solid media.
The isolation of enzymes from infected tissue can also provide the basis of a biochemical diagnosis of an infectious disease. For example, humans can make neither RNA replicases nor reverse transcriptase , and the presence of these enzymes are characteristic of specific types of viral infections. The ability of the viral protein hemagglutinin to bind red blood cells together into a detectable matrix may also be characterized as a biochemical test for viral infection, although strictly speaking hemagglutinin is not an enzyme and has no metabolic function.
Serological methods are highly sensitive, specific and often extremely rapid tests used to identify microorganisms. These tests are based upon the ability of an antibody to bind specifically to an antigen. The antigen, usually a protein or carbohydrate made by an infectious agent, is bound by the antibody.
This binding then sets off a chain of events that can be visibly obvious in various ways, dependent upon the test. For example, " Strep throat " is often diagnosed within minutes, and is based on the appearance of antigens made by the causative agent, S. Serological tests, if available, are usually the preferred route of identification, however the tests are costly to develop and the reagents used in the test often require refrigeration. Some serological methods are extremely costly, although when commonly used, such as with the "strep test", they can be inexpensive. Complex serological techniques have been developed into what are known as Immunoassays.
Immunoassays can use the basic antibody — antigen binding as the basis to produce an electro-magnetic or particle radiation signal, which can be detected by some form of instrumentation. Signal of unknowns can be compared to that of standards allowing quantitation of the target antigen. To aid in the diagnosis of infectious diseases, immunoassays can detect or measure antigens from either infectious agents or proteins generated by an infected organism in response to a foreign agent. For example, immunoassay A may detect the presence of a surface protein from a virus particle.
Immunoassay B on the other hand may detect or measure antibodies produced by an organism's immune system that are made to neutralize and allow the destruction of the virus. Instrumentation can be used to read extremely small signals created by secondary reactions linked to the antibody — antigen binding. Instrumentation can control sampling, reagent use, reaction times, signal detection, calculation of results, and data management to yield a cost effective automated process for diagnosis of infectious disease. Technologies based upon the polymerase chain reaction PCR method will become nearly ubiquitous gold standards of diagnostics of the near future, for several reasons.
First, the catalog of infectious agents has grown to the point that virtually all of the significant infectious agents of the human population have been identified. Second, an infectious agent must grow within the human body to cause disease; essentially it must amplify its own nucleic acids in order to cause a disease. This amplification of nucleic acid in infected tissue offers an opportunity to detect the infectious agent by using PCR. Third, the essential tools for directing PCR, primers , are derived from the genomes of infectious agents, and with time those genomes will be known, if they are not already.
Thus, the technological ability to detect any infectious agent rapidly and specifically are currently available. The only remaining blockades to the use of PCR as a standard tool of diagnosis are in its cost and application, neither of which is insurmountable. The diagnosis of a few diseases will not benefit from the development of PCR methods, such as some of the clostridial diseases tetanus and botulism.
These diseases are fundamentally biological poisonings by relatively small numbers of infectious bacteria that produce extremely potent neurotoxins. A significant proliferation of the infectious agent does not occur, this limits the ability of PCR to detect the presence of any bacteria. Given the wide range of bacteria, viruses, and other pathogens that cause debilitating and life-threatening illness, the ability to quickly identify the cause of infection is important yet often challenging.
For example, more than half of cases of encephalitis , a severe illness affecting the brain, remain undiagnosed, despite extensive testing using state-of-the-art clinical laboratory methods. Metagenomics is currently being researched for clinical use, and shows promise as a sensitive and rapid way to diagnose infection using a single all-encompassing test.
This test is similar to current PCR tests; however, amplification of genetic material is unbiased rather than using primers for a specific infectious agent. This amplification step is followed by next-generation sequencing and alignment comparisons using large databases of thousands of organismic and viral genomes. Metagenomic sequencing could prove especially useful for diagnosis when the patient is immunocompromised.
An ever-wider array of infectious agents can cause serious harm to individuals with immunosuppression, so clinical screening must often be broader. Additionally, the expression of symptoms is often atypical, making clinical diagnosis based on presentation more difficult. Thirdly, diagnostic methods that rely on the detection of antibodies are more likely to fail. A broad, sensitive test for pathogens that detects the presence of infectious material rather than antibodies is therefore highly desirable. There is usually an indication for a specific identification of an infectious agent only when such identification can aid in the treatment or prevention of the disease, or to advance knowledge of the course of an illness prior to the development of effective therapeutic or preventative measures.
For example, in the early s, prior to the appearance of AZT for the treatment of AIDS , the course of the disease was closely followed by monitoring the composition of patient blood samples, even though the outcome would not offer the patient any further treatment options. In part, these studies on the appearance of HIV in specific communities permitted the advancement of hypotheses as to the route of transmission of the virus. By understanding how the disease was transmitted, resources could be targeted to the communities at greatest risk in campaigns aimed at reducing the number of new infections.
The specific serological diagnostic identification, and later genotypic or molecular identification, of HIV also enabled the development of hypotheses as to the temporal and geographical origins of the virus, as well as a myriad of other hypothesis. Molecular diagnostics are now commonly used to identify HIV in healthy people long before the onset of illness and have been used to demonstrate the existence of people who are genetically resistant to HIV infection.
Thus, while there still is no cure for AIDS, there is great therapeutic and predictive benefit to identifying the virus and monitoring the virus levels within the blood of infected individuals, both for the patient and for the community at large. Techniques like hand washing, wearing gowns, and wearing face masks can help prevent infections from being passed from one person to another.
Aseptic technique was introduced in medicine and surgery in the late 19th century and greatly reduced the incidence of infections caused by surgery. Frequent hand washing remains the most important defense against the spread of unwanted organisms. Cooking foods well and avoiding foods that have been left outside for a long time is also important.
Antimicrobial substances used to prevent transmission of infections include:. One of the ways to prevent or slow down the transmission of infectious diseases is to recognize the different characteristics of various diseases. The human strains of Ebola virus, for example, incapacitate their victims extremely quickly and kill them soon after.
As a result, the victims of this disease do not have the opportunity to travel very far from the initial infection zone. Thus, the initial stage of Ebola is not very contagious since its victims experience only internal hemorrhaging. As a result of the above features, the spread of Ebola is very rapid and usually stays within a relatively confined geographical area.
Also, the relatively low virulence allows its victims to travel long distances, increasing the likelihood of an epidemic. Another effective way to decrease the transmission rate of infectious diseases is to recognize the effects of small-world networks. Despite the low interaction between discrete hubs, the disease can jump to and spread in a susceptible hub via a single or few interactions with an infected hub.
Thus, infection rates in small-world networks can be reduced somewhat if interactions between individuals within infected hubs are eliminated Figure 1. However, infection rates can be drastically reduced if the main focus is on the prevention of transmission jumps between hubs.
The use of needle exchange programs in areas with a high density of drug users with HIV is an example of the successful implementation of this treatment method. A general method to prevent transmission of vector -borne pathogens is pest control. Infection with most pathogens does not result in death of the host and the offending organism is ultimately cleared after the symptoms of the disease have waned.
Immunity mediated by these two factors may be manifested by:. The immune system response to a microorganism often causes symptoms such as a high fever and inflammation , and has the potential to be more devastating than direct damage caused by a microbe. You may be asked to quarantine yourself at home if you have developed, or been exposed to, an infectious disease If you want to get a tattoo, choose an experienced, registered practitioner to reduce the risks of infection and scarring Immunisation is one of the best ways you can protect yourself and others from infectious diseases in our community.
We all love travelling to new and exotic places, but unfortunately illnesses and unforeseen events can ruin the trip of a lifetime. With a little effort, take a few of these simple precautions to make If you are travelling overseas, check with your doctor well in advance to find out what immunisations you need Vaccines trick the body into building immunity against infectious diseases without causing the actual disease Some areas of the body are more susceptible to boils, including the face, throat, armpits, groin and buttocks Chickenpox is highly contagious, but it is generally mild and gets better without the need for special treatment Coughing and wheezing in babies can be distressing for you and your baby, but in most cases symptoms can be relieved at home Croup is a viral infection of the throat and windpipe that causes noisy breathing, a hoarse voice and a harsh, barking cough Diphtheria is a serious bacterial disease that causes severe inflammation of the nose, throat and windpipe Fever is a way in which the body fights infection.
A fever is not dangerous and does not always indicate a serious illness Gastroenteritis or Gastro can be dangerous for very young babies. Gastro is common in young children and spreads easily. Gastro is a bowel infection which causes diarrhoea runny or watery poo and Good personal hygiene is essential to prevent the spread of hand, foot and mouth disease to others, both for those infected and their carers Impetigo, or school sores, is a highly contagious skin infection that commonly affects school-aged children Measles can cause serious and sometimes fatal complications, including pneumonia and brain inflammation Immunisation is the best protection against measles, mumps, rubella and varicella chickenpox Roseola is a mild viral infection with associated fever and rash that affects babies and young children Rubella is a mild illness for most people, but very dangerous for pregnant women and their babies Children with slapped cheek do not need to be kept home from school or day care, as cases are only contagious before the onset of the rash Group B streptococcal bacteria can cause a wide range of illnesses in susceptible people including newborns, the elderly and those with pre-existing medical conditions such as diabetes or cancer.
Viral encephalitis is inflammation of the brain caused by a virus and can cause permanent brain damage The major symptom of whooping cough is a severe cough, which is often followed by a 'whooping' sound A family shares their experience when their baby daughter contracted whooping cough or pertussis Barmah Forest virus BFV disease can cause joint inflammation and pain, fatigue and a rash of variable appearance.
A full recovery can be expected. Most people recover completely within six months Leptospirosis is a disease spread from animals to humans, caused by infection with the bacteria Leptospira Travellers who visit malarial locations should avoid mosquito bites and take anti-malarial drugs You can reduce the risk of mosquito bites if you get rid of potential mosquito breeding sites around your home Despite the unsavoury reputation, a pinworm infection worms is relatively harmless and easily treated People who have birds as pets, poultry workers and anyone working in aviaries or pet shops, are most at risk of catching psittacosis Most people recover from Ross River virus disease, although some people have symptoms for a year or more Problems only occur if a woman becomes infected with parasites that cause toxoplasmosis for the first time while pregnant Zika virus is a mosquito-borne virus.
There is no cure, specific treatment or vaccine for Zika virus Aspergillus is a fungus that commonly grows on rotting vegetation. It can cause asthma symptoms Bacterial vaginosis BV is caused by an imbalance of the bacteria normally present in the vagina Hear from Victoria's Chief Health Officer talk about how you can protect yourself from mosquitoes this season If you are bitten or stung by an insect or animal, apply first aid and seek medical treatment as soon as possible Since the ulcer gets bigger with time, early diagnosis and prompt treatment of Bairnsdale disease can keep skin loss to a minimum Candida is a genus of fungi yeasts that live on the skin and inside the human body.
Candida auris also called C. Cellulitis is a bacterial infection of the skin that occurs most commonly on the lower legs and in areas where the skin is damaged or inflamed Chlamydia is often called the 'silent infection' because most people do not realise they have it If an unborn baby gets CMV from their mother, it can cause hearing loss and intellectual disability Listeria infection is uncommon but very dangerous for the elderly, people whose immune systems are not working properly and pregnant women and their unborn babies It is important to establish the cause of gastro, as different types of gastroenteritis respond to different treatments Campylobacteriosis is a type of gastroenteritis and is more common in children under five years of age and young adults Outbreaks of cryptosporidiosis have been associated with child care centres, public swimming pools and contaminated water supplies Most people infected with Giardia parasites do not develop symptoms but can still spread the infection to others You may be more prone to salmonellosis if you are elderly, have another medical condition such as a weakened immune system or are malnourished Outbreaks of shigella gastroenteritis can occur in institutional settings, particularly where children are still in nappies or adults are incontinent Many people with genital herpes are not aware that they have the infection, because they have no symptoms Glandular fever is most common among high school and university students, but young children can also become infected by saliva on toys, shared cups, or the hands of carers Gonorrhoea, also spelt gonorrhea, affects both men and women and is transmitted during sex, it may lead to infertility in women if left untreated Haemolytic uraemic syndrome HUS is a rare condition which can lead to chronic kidney damage or death from kidney failure Immunisation is the best protection against hepatitis A infection and it is recommended for people in high-risk groups Hepatitis B is a viral infection that affects the liver and can lead to serious illness or death In Australia, hepatitis C is most often spread through the sharing of unsterile drug injecting equipment.
New all oral combination treatment has greatly improved health outcomes for people with In Australia, HIV is most commonly spread when having sex without a condom and when sharing needles and other injecting equipment Labyrinthitis and vestibular neuritis are disorders that result in inflammation of the inner ear and the nerve connecting the inner ear to the brain Do not leave young adults alone if they suddenly develop a fever because they may become seriously ill very quickly Molluscum contagiosum can be mistaken for genital warts or pimples, check with your doctor for an accurate diagnosis Good personal hygiene is essential to prevent the spread of parechovirus to others, both for those infected and their carers Pericarditis symptoms may be similar to those of heart attack and include chest pain and abnormal heart rhythms Treating any infection of the upper respiratory tract quickly will reduce the risk of developing pleurisy Reactive arthritis is a form of arthritis that occurs as a result of some bacterial infections Rotavirus is a common cause of viral gastroenteritis for Australian babies and preschool children Salpingitis is one of the most common causes of female infertility and may permanently damage the fallopian tubes Bacteria in the bowels, urinary tract, mouth and skin can cause disease if they get into the bloodstream There are many types of E.
However, some types of E. One of these types of E. Hospital patients are more likely to be infected by golden staph because of surgical or other wounds Swimmer's ear can be triggered by exposure to water or mechanical damage due to overzealous cleaning Tinea is contagious and can be spread by skin-to-skin contact or indirectly through towels, clothes or floors If you think you could have toxic shock syndrome, stop using tampons immediately and go to the emergency department of your nearest hospital If you are travelling or passing through areas infected with yellow fever, some countries require you to be vaccinated Tuberculosis is spread when a person with an active infection coughs, laughs, sings or sneezes Viral haemorrhagic fever VHF is an extremely infectious and life threatening disease caused by a group of viruses, including the Ebola virus Williams syndrome often goes undiagnosed, which means that some people with the disorder fail to get the support and treatment they need until later in life Influenza the flu is caused by a virus.
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Each of the links must be present in a chronological order for an infection to develop. Understanding these steps helps health care workers target the infection and prevent it from occurring in the first place. Infection begins when an organism successfully enters the body, grows and multiplies. This is referred to as colonization. Most humans are not easily infected.
Those who are weak, sick, malnourished, have cancer or are diabetic have increased susceptibility to chronic or persistent infections. Individuals who have a suppressed immune system are particularly susceptible to opportunistic infections. Entrance to the host at host-pathogen interface , generally occurs through the mucosa in orifices like the oral cavity , nose, eyes, genitalia, anus, or the microbe can enter through open wounds.
While a few organisms can grow at the initial site of entry, many migrate and cause systemic infection in different organs. Some pathogens grow within the host cells intracellular whereas others grow freely in bodily fluids. Wound colonization refers to nonreplicating microorganisms within the wound, while in infected wounds, replicating organisms exist and tissue is injured.
All multicellular organisms are colonized to some degree by extrinsic organisms, and the vast majority of these exist in either a mutualistic or commensal relationship with the host. An example of the former is the anaerobic bacteria species, which colonizes the mammalian colon , and an example of the latter are the various species of staphylococcus that exist on human skin.
Neither of these colonizations are considered infections. The difference between an infection and a colonization is often only a matter of circumstance. Non-pathogenic organisms can become pathogenic given specific conditions, and even the most virulent organism requires certain circumstances to cause a compromising infection. Some colonizing bacteria, such as Corynebacteria sp. The variables involved in the outcome of a host becoming inoculated by a pathogen and the ultimate outcome include:. As an example, several staphylococcal species remain harmless on the skin, but, when present in a normally sterile space, such as in the capsule of a joint or the peritoneum , multiply without resistance and cause harm.
An interesting fact that gas chromatography—mass spectrometry , 16S ribosomal RNA analysis, omics , and other advanced technologies have made more apparent to humans in recent decades is that microbial colonization is very common even in environments that humans think of as being nearly sterile. Because it is normal to have bacterial colonization, it is difficult to know which chronic wounds can be classified as infected and how much risk of progression exists. Despite the huge number of wounds seen in clinical practice, there are limited quality data for evaluated symptoms and signs.
A review of chronic wounds in the Journal of the American Medical Association's "Rational Clinical Examination Series" quantified the importance of increased pain as an indicator of infection. Disease can arise if the host's protective immune mechanisms are compromised and the organism inflicts damage on the host.
Microorganisms can cause tissue damage by releasing a variety of toxins or destructive enzymes. For example, Clostridium tetani releases a toxin that paralyzes muscles, and staphylococcus releases toxins that produce shock and sepsis. Not all infectious agents cause disease in all hosts. The prion causing mad cow disease and Creutzfeldt—Jakob disease invariably kills all animals and people that are infected. Persistent infections occur because the body is unable to clear the organism after the initial infection.
Persistent infections are characterized by the continual presence of the infectious organism, often as latent infection with occasional recurrent relapses of active infection. There are some viruses that can maintain a persistent infection by infecting different cells of the body. Some viruses once acquired never leave the body. A typical example is the herpes virus, which tends to hide in nerves and become reactivated when specific circumstances arise.
Persistent infections cause millions of deaths globally each year. For infecting organisms to survive and repeat the infection cycle in other hosts, they or their progeny must leave an existing reservoir and cause infection elsewhere. Infection transmission can take place via many potential routes:. The relationship between virulence versus transmissibility is complex; if a disease is rapidly fatal, the host may die before the microbe can be passed along to another host.
Diagnosis of infectious disease sometimes involves identifying an infectious agent either directly or indirectly. In practice most minor infectious diseases such as warts , cutaneous abscesses , respiratory system infections and diarrheal diseases are diagnosed by their clinical presentation and treated without knowledge of the specific causative agent. Conclusions about the cause of the disease are based upon the likelihood that a patient came in contact with a particular agent, the presence of a microbe in a community, and other epidemiological considerations.
Given sufficient effort, all known infectious agents can be specifically identified. The benefits of identification, however, are often greatly outweighed by the cost, as often there is no specific treatment, the cause is obvious, or the outcome of an infection is benign.
Diagnosis of infectious disease is nearly always initiated by medical history and physical examination. More detailed identification techniques involve the culture of infectious agents isolated from a patient. Culture allows identification of infectious organisms by examining their microscopic features, by detecting the presence of substances produced by pathogens, and by directly identifying an organism by its genotype.
The images are useful in detection of, for example, a bone abscess or a spongiform encephalopathy produced by a prion. The diagnosis is aided by the presenting symptoms in any individual with an infectious disease, yet it usually needs additional diagnostic techniques to confirm the suspicion. Some signs are specifically characteristic and indicative of a disease and are called pathognomonic signs; but these are rare.
Not all infections are symptomatic. In children the presence of cyanosis , rapid breathing, poor peripheral perfusion, or a petechial rash increases the risk of a serious infection by greater than 5 fold. Microbiological culture is a principal tool used to diagnose infectious disease.
In a microbial culture, a growth medium is provided for a specific agent. A sample taken from potentially diseased tissue or fluid is then tested for the presence of an infectious agent able to grow within that medium.
Most pathogenic bacteria are easily grown on nutrient agar , a form of solid medium that supplies carbohydrates and proteins necessary for growth of a bacterium , along with copious amounts of water. A single bacterium will grow into a visible mound on the surface of the plate called a colony , which may be separated from other colonies or melded together into a "lawn".
The size, color, shape and form of a colony is characteristic of the bacterial species, its specific genetic makeup its strain , and the environment that supports its growth. Other ingredients are often added to the plate to aid in identification. Plates may contain substances that permit the growth of some bacteria and not others, or that change color in response to certain bacteria and not others.
Bacteriological plates such as these are commonly used in the clinical identification of infectious bacterium. Microbial culture may also be used in the identification of viruses: In the case of viral identification, a region of dead cells results from viral growth, and is called a "plaque". Eukaryotic parasites may also be grown in culture as a means of identifying a particular agent.
In the absence of suitable plate culture techniques, some microbes require culture within live animals. Bacteria such as Mycobacterium leprae and Treponema pallidum can be grown in animals, although serological and microscopic techniques make the use of live animals unnecessary. Viruses are also usually identified using alternatives to growth in culture or animals. Some viruses may be grown in embryonated eggs. Another useful identification method is Xenodiagnosis, or the use of a vector to support the growth of an infectious agent. Chagas disease is the most significant example, because it is difficult to directly demonstrate the presence of the causative agent, Trypanosoma cruzi in a patient, which therefore makes it difficult to definitively make a diagnosis.
In this case, xenodiagnosis involves the use of the vector of the Chagas agent T. The bug is later inspected for growth of T. Another principal tool in the diagnosis of infectious disease is microscopy. Virtually all of the culture techniques discussed above rely, at some point, on microscopic examination for definitive identification of the infectious agent. Microscopy may be carried out with simple instruments, such as the compound light microscope , or with instruments as complex as an electron microscope.
Samples obtained from patients may be viewed directly under the light microscope, and can often rapidly lead to identification. Microscopy is often also used in conjunction with biochemical staining techniques, and can be made exquisitely specific when used in combination with antibody based techniques. For example, the use of antibodies made artificially fluorescent fluorescently labeled antibodies can be directed to bind to and identify a specific antigens present on a pathogen.
A fluorescence microscope is then used to detect fluorescently labeled antibodies bound to internalized antigens within clinical samples or cultured cells. This technique is especially useful in the diagnosis of viral diseases, where the light microscope is incapable of identifying a virus directly. Other microscopic procedures may also aid in identifying infectious agents. Almost all cells readily stain with a number of basic dyes due to the electrostatic attraction between negatively charged cellular molecules and the positive charge on the dye.
A cell is normally transparent under a microscope, and using a stain increases the contrast of a cell with its background. Staining a cell with a dye such as Giemsa stain or crystal violet allows a microscopist to describe its size, shape, internal and external components and its associations with other cells. The response of bacteria to different staining procedures is used in the taxonomic classification of microbes as well.
Two methods, the Gram stain and the acid-fast stain, are the standard approaches used to classify bacteria and to diagnosis of disease. The Gram stain identifies the bacterial groups Firmicutes and Actinobacteria , both of which contain many significant human pathogens. The acid-fast staining procedure identifies the Actinobacterial genera Mycobacterium and Nocardia. Biochemical tests used in the identification of infectious agents include the detection of metabolic or enzymatic products characteristic of a particular infectious agent.
Since bacteria ferment carbohydrates in patterns characteristic of their genus and species , the detection of fermentation products is commonly used in bacterial identification. Acids , alcohols and gases are usually detected in these tests when bacteria are grown in selective liquid or solid media. The isolation of enzymes from infected tissue can also provide the basis of a biochemical diagnosis of an infectious disease. For example, humans can make neither RNA replicases nor reverse transcriptase , and the presence of these enzymes are characteristic of specific types of viral infections.
The ability of the viral protein hemagglutinin to bind red blood cells together into a detectable matrix may also be characterized as a biochemical test for viral infection, although strictly speaking hemagglutinin is not an enzyme and has no metabolic function. Serological methods are highly sensitive, specific and often extremely rapid tests used to identify microorganisms.
These tests are based upon the ability of an antibody to bind specifically to an antigen. The antigen, usually a protein or carbohydrate made by an infectious agent, is bound by the antibody. This binding then sets off a chain of events that can be visibly obvious in various ways, dependent upon the test. For example, " Strep throat " is often diagnosed within minutes, and is based on the appearance of antigens made by the causative agent, S.
Serological tests, if available, are usually the preferred route of identification, however the tests are costly to develop and the reagents used in the test often require refrigeration. Some serological methods are extremely costly, although when commonly used, such as with the "strep test", they can be inexpensive. Complex serological techniques have been developed into what are known as Immunoassays.
Immunoassays can use the basic antibody — antigen binding as the basis to produce an electro-magnetic or particle radiation signal, which can be detected by some form of instrumentation. Signal of unknowns can be compared to that of standards allowing quantitation of the target antigen. To aid in the diagnosis of infectious diseases, immunoassays can detect or measure antigens from either infectious agents or proteins generated by an infected organism in response to a foreign agent. For example, immunoassay A may detect the presence of a surface protein from a virus particle.
Immunoassay B on the other hand may detect or measure antibodies produced by an organism's immune system that are made to neutralize and allow the destruction of the virus. Instrumentation can be used to read extremely small signals created by secondary reactions linked to the antibody — antigen binding. Instrumentation can control sampling, reagent use, reaction times, signal detection, calculation of results, and data management to yield a cost effective automated process for diagnosis of infectious disease.