Medical microbiology , the large subset of microbiologythat is appliedto medicine, is a branch of medical science concerned with the prevention, diagnosis and treatment of infectious diseases. In addition, this field of science studies various clinical applications of microbes for the improvement of health. There are four kinds of microorganismsthat cause infectious disease: bacteria, fungi, parasitesand viruses, and one type of infectious protein called prion.
A medical microbiologiststudies the characteristics of pathogens, their modes of transmission, mechanisms of infection and growth.Using this information, a treatment can be devised. Medical microbiologists often serve as consultants for physicians, providing identification of pathogens and suggesting treatment options. Other tasks may include the identification of potential health risks to the community or monitoring the evolution of potentially virulentor resistant strains of microbes, educating the community and assisting in the design of health practices. They may also assist in preventing or controlling epidemicsand outbreaks of disease. Not all medical microbiologists study microbial pathology; some study common, non-pathogenic species to determine whether their properties can be used to develop antibioticsor other treatment methods.
Epidemiology, the study of the patterns, causes, and effects of healthand diseaseconditions in populations, is an important part of medical microbiology, although the clinical aspect of the field primarily focuses on the presence and growth of microbial infections in individuals, their effects on the human body, and the methods of treating those infections. In this respect the entire field, as an applied science, can be conceptually subdivided into academic and clinical subspecialties, although in reality there is a fluid continuum between public health microbiology and clinical microbiology, just as the state of the art in clinical laboratoriesdepends on continual improvements in academic medicine and research laboratories.
In 1676, Anton van Leeuwenhoekobserved bacteria and other microorganisms, using a single-lens microscopeof his own design.
In 1796, Edward Jennerdeveloped a method using cowpoxto successfully immunize a child against smallpox. The same principles are used for developing vaccinestoday.
Following on from this, in 1857 Louis Pasteuralso designed vaccines against several diseases such as anthrax, fowl choleraand rabiesas well as pasteurizationfor food preservation.
In 1867 Joseph Listeris considered to be the father of antisepticsurgery. By sterilizing the instruments with diluted carbolic acidand using it to clean wounds, post-operative infections were reduced, making surgery safer for patients.
In the years between 1876 and 1884 Robert Kochprovided much insight into infectious diseases. He was one of the first scientists to focus on the isolation of bacteria in pure culture. This gave rise to the germ theory, a certain microorganism being responsible for a certain disease. He developed a series of criteria around this that have become known as the Koch’s postulates.
A major milestone in medical microbiology is the Gram stain. In 1884 Hans Christian Gramdeveloped the method of staining bacteria to make them more visible and differentiable under a microscope. This technique is widely used today.
In 1929 Alexander Flemingdeveloped the most commonly used antibiotic substance both at the time and now: penicillin.
DNA sequencing, a method developed by Walter Gilbertand Frederick Sangerin 1977,caused a rapid change the development of vaccines, medical treatments and diagnostic methods. Some of these include synthetic insulinwhich was produced in 1979 using recombinant DNAand the first genetically engineered vaccine was created in 1986 for hepatitis B.
In 1995 a team at The Institute for Genomic Researchsequenced the first bacterial genome; Haemophilus influenzae.A few months later, the first eukaryoticgenome was completed. This would prove invaluable for diagnostic techniques. Streptococcal pharyngitis Chlamydia Typhoid fever Tuberculosis Rotavirus Hepatitis C[13 Human papillomavirus(HPV) Malaria Giardia lamblia Toxoplasma gondii Candida Histoplasmosis
Infections may be caused by bacteria, viruses, fungi, and parasites. The pathogen that causes the disease may be exogenous (acquired from an external source; environmental, animal or other people, e.g. Influenza) or endogenous (from normal flora e.g. candidiasis).
The site at which a microbe enters the body is referred to as the portal of entry.These include the respiratory tract, gastrointestinal tract, genitourinary tract, skin, and mucous membranes.The portal of entry for a specific microbe is normally dependent on how it travels from its natural habitat to the host. There are various ways in which disease can be transmitted between individuals. These include:
Direct contact – Touching an infected host, including sexual contact
Indirect contact – Touching a contaminated surface
Droplet contact- Coughing or sneezing
Fecal–oral route- Ingesting contaminated food or water sources
Airborne transmission – Pathogen carrying spores
Vector transmission- An organism that does not cause disease itself but transmits infection by conveying pathogens from one host to another
Fomite transmission- An inanimate object or substance capable of carrying infectious germs or parasites
Environmental – Hospital-acquired infection (Nosocomial infections)
Like other pathogens, viruses use these methods of transmission to enter the body, but viruses differ in that they must also enter into the host’s actual cells. Once the virus has gained access to the host’s cells, the virus’ genetic material (RNAor DNA) must be introduced to the cell. Replication between viruses is greatly varied and depends on the type of genes involved in them. Most DNA viruses assemble in the nucleus while most RNA viruses develop solely in cytoplasm.
The mechanisms for infection, proliferation, and persistence of a virus in cells of the host are crucial for its survival. For example, some diseases such as measlesemploy a strategy whereby it must spread to a series of hosts. In these forms of viral infection, the illness is often treated by the body’s own immune response, and therefore the virus is required to disperse to new hosts before it is destroyed by immunological resistanceor host death.In contrast, some infectious agents such as the Feline leukemia virus, are able to withstand immune responses and are capable of achieving long-term residence within an individual host, whilst also retaining the ability to spread into successive hosts.
Identification of an infectious agent for a minor illness can be as simple as clinical presentation; such as gastrointestinal diseaseand skin infections. In order to make an educated estimate as to which microbe could be causing the disease, epidemiological factors need to be considered; such as the patient’s likelihood of exposure to the suspected organism and the presence and prevalence of a microbial strain in a community.
Diagnosis of infectious disease is nearly always initiated by consulting the patient’s medical history and conducting a physical examination. More detailed identification techniques involve microbial culture, microscopy, biochemical testsand genotyping. Other less common techniques (such as X-rays, CAT scans, PET scansor NMR) are used to produce images of internal abnormalities resulting from the growth of an infectious agent.
Microbiological cultureis the primary method used for isolating infectious disease for study in the laboratory. Tissue or fluid samples are tested for the presence of a specific pathogen, which is determined by growth in a selective or differential medium.
The 3 main types of media used for testing are: Solid culture: A solid surface is created using a mixture of nutrients, salts and agar. A single microbe on an agar plate can then grow into colonies (clones where cells are identical to each other) containing thousands of cells. These are primarily used to culture bacteria and fungi. Liquid culture: Cells are grown inside a liquid media. Microbial growth is determined by the time taken for the liquid to form a colloidal suspension. This technique is used for diagnosing parasites and detecting mycobacteria. Cell culture: Human or animal cell culturesare infected with the microbe of interest. These cultures are then observed to determine the effect the microbe has on the cells. This technique is used for identifying viruses.
Culture techniqueswill often use a microscopic examination to help in the identification of the microbe. Instruments such as compound light microscopescan be used to assess critical aspects of the organism. This can be performed immediately after the sample is taken from the patient and is used in conjunction with biochemical staining techniques, allowing for resolution of cellular features. Electron microscopesand fluorescence microscopesare also used for observing microbes in greater detail for research.
Fast and relatively simple biochemical testscan be used to identify infectious agents. For bacterial identification, the use of metabolicor enzymatic characteristics are common due to their ability to ferment carbohydratesin patterns characteristic of their genusand species. Acids, alcohols and gases are usually detected in these tests when bacteria are grown in selective liquid or solid media, as mentioned above. In order to perform these tests en masse, automated machines are used. These machines perform multiple biochemical tests simultaneously, using cards with several wells containing different dehydrated chemicals. The microbe of interest will react with each chemical in a specific way, aiding in its identification.
Serologicalmethods are highly sensitive, specific and often extremely rapid laboratory tests used to identify different types of microorganisms. The tests are based upon the ability of an antibodyto bind specifically to an antigen. The antigen (usually a protein or carbohydrate made by an infectious agent) is bound by the antibody, allowing this type of test to be used for organisms other than bacteria. This binding then sets off a chain of events that can be easily and definitively observed, depending on the test. More complex serological techniques are known as immunoassays. Using a similar basis as described above, immunoassays can detect or measure antigens from either infectious agents or the proteins generated by an infected host in response to the infection.
Polymerase chain reaction(PCR) assays are the most commonly used molecular technique to detect and study microbes.As compared to other methods, sequencing and analysis is definitive, reliable, accurate, and fast.Today, quantitative PCRis the primary technique used, as this method provides faster data compared to a standard PCR assay. For instance, traditional PCR techniques require the use of gel electrophoresisto visualize amplified DNA molecules after the reaction has finished. quantitative PCRdoes not require this, as the detection system uses fluorescenceand probesto detect the DNA molecules as they are being amplified.In addition to this, quantitative PCRalso removes the risk of contamination that can occur during standard PCR procedures (carrying over PCR product into subsequent PCRs).Another advantage of using PCR to detect and study microbes is that the DNA sequences of newly discovered infectious microbes or strains can be compared to those already listed in databases, which in turn helps to increase understanding of which organism is causing the infectious disease and thus what possible methods of treatment could be used.This technique is the current standard for detecting viral infections such as AIDSand hepatitis.
Once an infection has been diagnosed and identified, suitable treatment options must be assessed by the physician and consulting medical microbiologists. Some infections can be dealt with by the body’s own immune system, but more serious infections are treated with antimicrobialdrugs. Bacterial infectionsare treated with antibacterials(often called antibiotics) whereas fungaland viralinfections are treated with antifungalsand antiviralsrespectively. A broad class of drugs known as antiparasiticsare used to treat parasitic diseases.
Medical microbiologists often make treatment recommendations to the patient’s physician based on the strain of microbeand its antibiotic resistances, the site of infection, the potential toxicityof antimicrobial drugs and any drug allergiesthe patient has.
In addition to drugs being specific to a certain kind of organism (bacteria, fungi, etc.), some drugs are specific to a certain genusor speciesof organism, and will not work on other organisms. Because of this specificity, medical microbiologists must consider the effectiveness of certain antimicrobial drugs when making recommendations. Additionally, strainsof an organism may be resistant to a certain drug or class of drug, even when it is typically effective against the species. These strains, termed resistant strains, present a serious public health concern of growing importance to the medical industry as the spread of antibiotic resistanceworsens. Antimicrobial resistanceis an increasingly problematic issue that leads to millions of deaths every year.
Whilst drug resistance typically involves microbes chemically inactivating an antimicrobial drug or a cell mechanically stopping the uptake of a drug, another form of drug resistance can arise from the formation of biofilms. Some bacteria are able to form biofilms by adhering to surfaces on implanted devices such as catheters and prostheses and creating an extracellular matrixfor other cells to adhere to.This provides them with a stable environment from which the bacteria can disperse and infect other parts of the host. Additionally, the extracellular matrix and dense outer layer of bacterial cells can protect the inner bacteria cells from antimicrobial drugs.
Medical microbiology is not only about diagnosing and treating disease, it also involves the study of beneficial microbes. Microbes have been shown to be helpful in combating infectious disease and promoting health. Treatments can be developed from microbes, as demonstrated by Alexander Fleming’s discovery of penicillinas well as the development of new antibiotics from the bacterial genus Streptomycesamong many others.Not only are microorganisms a source of antibiotics but some may also act as probioticsto provide health benefits to the host, such as providing better gastrointestinal health or inhibiting pathogens.
Text under construction
- Clinical pathology
- Fungal infection
- List of antibiotics
- List of human parasitic diseases
- List of infectious diseases
- Pathogenic bacteria
- Viral disease
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