Microbiology is the study of the biology of microscopic organisms - viruses, bacteria, algae, fungi, slime molds, and protozoa. The methods used to study and manipulate these minute and mostly unicellular organisms differ from those used in most other biological investigations.
1. B acteria
• A re prokaryotic (i.e. have no m em brane-bound organelles)
• Can be classified into 3 main groups (Box 9.1)
B ox 9.1 Bacterial groups
G ra m sta ina b le A cid -fa st b acilli U nusual
Gram positive • Cell w all has high lipid • Have no peptidoglycans
Gram negative content w hence d ifficult to (e.g. Chlamydia, Mycoplasma)
Gram variable (Gardenella stain (e.g. Mycobacteria,
vaginalis, Mobiluncus) :. Norcardia)
• A re visible by light m icroscopy (average diam eter = 1 pm)
• Have a cell wall which is m ade up o f
i. N-acetyl glucosamine/muramic acid
ii. Peptidoglycans (M u r e in )
■ Have penicillin-binding sites
■ A re th e targ et fo r [i-lactams
iii. Polypeptides
iv. Polysaccharides
2 . T axon om y
• By shape
i. Bacilli (rods)
ii. C o cci (grains)
• By 0 2 requirem ent
i. A erobes
ii. A naerobes
, 284 C h a p te r 9 Microbiology ' 1
• By sp o re forming
• By staining
3. Anaerobe organisms can be classified into 2 types
• Facultative anaerobes, which are capable o f aerobic respiration if 0 2 is present
• O bligate anaerobes, which die in th e presence o f 0 2
4. G ram stain
• Process involves
i. Staining with crystal violet
ii. Then staining with G ram ’s Iodine
iii. Decolourizing with aceton e
iv. Counter-stain with methyl red
• Gram -positive bacteria
i. Stain blue — retain crystal violet stain
ii. Stain due to peptidoglycan - a thick polysaccharide co at that loses stain very slowly once
taken up
iii. Include (Box 9.2)
Gram -negative bacteria (Fig. 9.1)
i. Stain pink because th e cell wall is thinner and does no t retain th e crystal violet dye, so it
takes up th e methyl red stain
ii. Cell wall consists o f
■ O u te r layer o f LPS
■ Periplasmic layer containing p-lactam ase
Figure 9.1 G ram -negative bacteria nom enclature
, ■ Inner peptidoglycan layer
iii. Gram -negative bacilli include
* Haemophilus influenzae
■ Klebsiella pneumonia
* Legionella
* Pseudomonas aenjginosa
* Escherichia coli
■ Proteus mirabilis
■ Helicobacter pylori
■ Salmonella typhi
■ Campylobacter
iv. Can also be divided based on lactose ferm entation (B o x 9.3)
B ox 9.3 Gram negative bacteria classification according to lactose ferm entation
Lactose fe rm e n te rs (o range on M c C o n ke y agar) L acto se n o n -fe rm e n te rs (p in k o n M cC o n k e y agar)
5. Bacterial toxins consist o f 2 types
• Exotoxins
i. Se creted by organisms
ii. A featu re o f Gram -positive and Gram -negative bacteria
iii. Form toxoids o
• Endotoxin
i. Released on cell death and lysis
ii. A feature o f Gram -negative bacteria
iii. T h e main form s is lipid A from LPS
6. Bacterial antimicrobial resistance occurs via
• Bacterial m echanisms o f antimicrobial resistance
i. Drug inactivation (e.g. production o f p-lactam ases)
ii. A lteration o f drug targ et site (e.g. alteration o f penicillin-binding sites)
iii. Bacterium m etabolic pathway alteration
iv. Fibronectin co at
v. IgA cleaving p rotease
• Mechanisms o f transfer o f antimicrobial resistance
i- Horizontal gene transfer
ii. Vertical gene transfer
• Mechanisms o f horizontal gene transfer
i- Plasmid DN A transfer
ii. C hrom osom al mediated resistance
iii. Bacterial conjugation
' Vaginal flo ra is influenced by o e stro g e n levels co n tribu tin g to
• Increased vaginal glycogen concentration
• pH 3 .S -4 .5 due to conversion o f glycogen to lactic acid by lactobacilli
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