Thursday, October 6, 2011

Classification & pathogenicity of Microbes

The microbial causes of human disease include viruses, Chlamydia, Richettsiae, Mycoplasmas, bacteria, fungi & protozoa.
Viruses differ greatly from all the other microbes as they consist essentially of only nucleic acid surrounded by a protein coat (capsid) & contain only one instead of two types of nucleic acid.
Chlamydiae & Richettsiae are also obligate intracellular parasites, have both DNA & RNA & multiply by binary fission. Mycoplasmas, bacteria & fungi can be cultured in cell free media unlike the above intra cellular microbes .
Protozoa pathogenic to man are divided into 3 main groups :
1. Sarcodina (amoebae) e g Entamoeba histolytica .
2. Sporozoa eg. Plasmodium falciparum, Toxoplasma gondii.
3. Mostigophora (flagellates) eg Trichomonas vaginalis, Giardia lambilia, Leishmania & Trypansoma sp.
Classification of bacteria :
There are 3 main group of bacteria :
1- Bacteria that are readily Gram – Stained
2- Acid – fast bacilli
3- Spirochetes'
Bacteria that are readily Gram – Stained:
These are classified into Gram positive (blue-purple) or Gram negative (pink-red) cocci or bacilli (Table 2)
After the application of the methyl violet dye, Gram positive bacteria stain blue & this color is retained in spite of decolorization with acetone (or alcohol)and Gram negative bacteria initially stain blue after the methyl violet is applied but the color is lost after the application of acetone (or alcohol). They then take up the pink counter stain (Carbol fuchsine, methyl red or saffronin).
The reason for the difference in color after gram staining is not fully understood, but it is probably related to the large amount of mucopeptide & teichoic acid in the cell walls of Gram positive bacteria. The fact that Gram positive bacteria are more acidic than Gram negative bacteria may account for their greater affinity for a basic dye. Even more important may be the greater permeability of Gram negative cell walls which allow the methyl violet – iodine dye complex to diffuse out after treatment with acetone more readily than cell walls of Gram positive bacteria.
Within each sub group, there are aerobic or anaerobic examples . The majority of bacterial pathogens can grow either aerobically or anaerobically i.e. facultative anaerobes such as Staphylococcus aureus or Escherichia coli in table 2 these have been included as aerobes. There are a few bacterial species which are strict aerobes such as Pseudomonas aeruginosa which will not grow at all anaerobically. Some bacterial species are strict anaerobes such as Clasteriduim tetani or Bacteroides fragilis, which will not grow at all aerobically .
Exceptional Gram negative stainable bacteria include Legionella pneumophillia & Borrelia vincent. Legionella pneumophillia requires prolonged staining with the counter stain to be seen in tissues, although it appears readily as Gram negative bacilli in smears made from colonies on agar media. Borrelia vincent is the only spirochete pathogen that is easily seen by a Gram stain.
Acid- fast bacilli :
Mycobacteria species are not readily seen by a Gram stain. Zheil-Nelseen ( ZN) or other acid- fast stains are required for staining these organisms which have cell walls containing abundant lipids. Examples include Mycobacterium tuberculosis & Mycobacterium leprae .
Spirochetes :
They are thin walled spiraled flexible organism which are motile by means of an axial filaments and they are not seen in a Gram stain (except Borrelia vincent) but may be seen either by dark ground illumination microscopy or in a silver stain under the light microscope. Borrelia spirochaetes in the blood may also be seen in a Giemsa stain.
The three groups of spirochetes include :
1. Treponema : Spirochetes with regular spirals, about 1um apart from each other, 5 – 15um long & about 0.2um wide e.g. Treponema pallidum .
2. Leptospira : have tightly spirals, 5 – 15um long & about 0.1um wide. There is often a hooked end eg Leptospira ictero haemorrhagiae (Weill's)
3. Borrelia : Large spirochaetes, 10 – 30um long & about 0.3um wide, with irregular spirals 2 – 4um apart from each other eg Borrelia recurrentis (a cause of relapsing fever.
Classification of viruses:
The classification of viruses depends on several factors including the type of nucleic acid present, the arrangement of the capsids into a circular(icosahedral), helical or complex symmetry, the number of capsomeres, the shape of the virus particle & whether the vision is naked or enveloped .
DNA viruses are:
Pox & papova, Herpes & adeno virus & the remaining viruses & the remaining viruses are RNA viruses .
Classification of fungi :
There are 4 main groups of pathogenic fungi moulels (filamentous fungi) true yeasts yeast – like fungi & dimorphic fungi
1- Filamentous fungi :
Theses grow as long filaments called hyphae & the branched hyphae intervene to form a mycelium. Reproduction is by spores including sexual spores which are used for identification. Culture on Sabaroud's powdery colonies due to the presence of spores eg Trichophyton mentagrophyets .
2- True yeasts :
These are unicellular round or oval fungi . Reproduction is by ludding from the parent cell . Cultures creamy colonies e.g. Cryptococcus neoformans .
3- Yeast like fungi:
These are like yeasts since they may appear as round or oval cells & grow by budding . They may also form long non branching filaments known as pseudohyphae e.g. Candida albicans.
4- Dimorphic fungi :
These grow as yeast forms in the body & at 37oC on culture media. They also form mycelia in the environment & on culture media at 22oC eg Histoplasma cupsulatum.
Fungi can also be classified according to whether they cause superficial or deep mycoses in infected patients .
Pathogenesis : Factors affecting the virulence & Spread of Microbes :
The pathogenicity of a microbe depends on host as well as on microbial factors . Host factors include the age of the patient, genetic factors, general host defenses & local host defenses .
Lock's postulates :
1. The particular microbe is always associated with a given disease .
2. The microbe may be isolated in the laboratory from specimens from a patient with the disease .
It is possible to produce a similar disease in animals by inoculation of the microbe into animals. Mycobacterium tuberculosis may be an example where these 3 postulates may be fulfilled.
Factors affecting "Virulence":
The main known factors that affect virulence are concerned with pathogenicity such as toxins & capsules in bacteria . It has also become apparent that the virulence of bacterial stains may also depend on the presence of transmissible genes contained in plasmids or mediated by bacteriophage.
The toxins produced by Corny diphtheriae & the erythrogenic toxin produced by Streptococcus pyogenes strains in scarlet fever patients are dependent on genes mediated by temperate phages. The fact that particular microbes appear to be more or less virulent at different times, might be due in part to the presence or absence of these types of transmissible genes.
Factors affecting spread:
Epidemiological factors affecting the host are relevant to the spread of microbes including the numbers of susceptible individuals in a geographically defined area, the proximity of the individual to each other & to the source of infection & the presence of other factors necessary for the transmission of infection such as the correct climate or season, the presence of an essential arthropod vector, etc.
Microbial factors that affect the spread depend partly on the virulence of the microbe & partly on the ability of the microbe to survive or multiply in a given inanimate environment or on the hands of patients or hospital staff or in animals arthropods. Carrier states clearly aid the transmission of bacteria. Gram positive bacteria survive well in dry environments while Gram negative & some spirochetes survive lest in moist situations .
Microbes are either transmitted horizontally i.e. between individuals of the same generation (such as the plague bacillus) or vertically i.e. between individuals of different generation (such as cong. Rubella for mother to infant).
Infection either endogenous from the patient's own flora or exogenous from a source outside such as another patient or person, an animal, a vehicle or fomite .
Mode of transmission of microbes include:
1. Direct contact such as N. gonorrhoea .
2. Ingestion such as V. cholerae.
3. Inoculation such as injury transmitting hepatitis B, mosquito bite transmitting malaria .
4. Inhalation such as with measles, Rhino viruses or Mycobacterium tuberculosis.
Lectures on applied clinical microbiology

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