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Principles of Bacteriology

Prepared byHamad ALAssaf

Alassaf_h@yahoo.com2015

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Bacterial Structure

Peptidoglycan : gives rigid support, protect against osmotic pressure.

Capsule: protect against phagocytosis. Polysaccharides except Bacillus anthrax, which contains D-glutamate.

Spore: provides resistance to dehydration, heat, and chemicals. Keratin-like coat; dipicolinic acid.

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Gram Stain Limitations

•These organism do not Gram stain well:1- Treponema (too thin to be visualized).2- Rickettsia (intracellular parasite).3- Mycobacteria (No cell wall).4- Legionella pnumophila ( primarily intracellular).5- Chlamydia (intracellular parasite; lacks muramic acid in cell wall).

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Stains

• Giemsa : Borrelia, Plasmodium, Trypanosomes, Chlamydia.

•PAS (periodic acid-Schiff): Stains glycogen, mucopolysaccharides; used to diagnose Whipple’s disease (Tropheryma whippelii).

•Ziehl-Neelsen: Acid-fast organism.

• Indian ink: Cryptococcus neoformans, and used to stain thick polysaccharide capsule red.

•Silver stain: Fungi (e.g. Pneumocystis), Legionella.

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Silver Stain Indian ink Giemsa Stain

ZN stain

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Special culture requirements

Microrganism

Media used for isolation

H. influenzae Chocolate agar with factors V (NAD) and X (hematin).

N. gonorrhoeae Thayer-Martin Media (VPN)B. pertussis Bordet-Gengou agarC. diphtheriae Tellurite plate, Loffler’s mediaM. tuberculosis Lowenstein-Jensen agarM. pneumoniae Eaton’s agarLegionella Charcoal yeast extract agar buffered with

cysteineFungi Sabouraud’s agarEnteric pathogens Hektone enteric agar or Xylose-Lysine-

Deoxycholate agarVibrio cholerae TCBS (alkaline growth medium)

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•Encapsulated Bacteria: Positive Quellung reaction e.g. Streptoccoccus pneumonia, Klebsiella pneumonia, Haemophilus influnesae type B, Neisseria meningitides, Salmonella, Group B streptococcus.•Urease-positive bugs: Proteus, Klebsiella, H.pylori, Ureaplasma.•Pigment-producing bacteria: Actinomyces israelii (yellow –sulfur), S.aureus (yellow pigment), Pseudomonas aeruginosa (blue-green pigment), Serratia marcescens (red pigment).

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Bacterial virulence factors

•These promote evasion of host immune response.

1- Protein A (S. aureus): Binds Fc region of Ig. Prevent opsonization and phagocytosis.2- IgA protease: Enzyme that cleaves IgA. Secreted by S.pneumoniae, H. influenza type B, and Neisseria in order to colonize respiratory mucosa.3- M protein (group A streptococcus): helps prevent phagocytosis.

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Normal Flora• Is found on body surfaces contiguous with the outside environment.

• Is semi-permanent, varying with major life changes.

• Can cause infection:

- if misplaced , e.g. fecal flora to urinary tract or abdominal cavity, or skin flora to catheter.

- if person become compromised, normal flora may overgrow (oral thrush).

• contribute to health:

- Protective host defense by maintaining conditions such as pH so other organism may not grow.

- Serves nutritional function by synthesizing: K and B12 vitamins. Normal Flora e.g. Nose (S.aureus), cutaneous (Staphylococcus epidermidis), Oropharynx

(Viridans streptococci), Vagina (Lactobacillus), colon (E.coli).Blood and Stomach: No Normal Flora present.

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Colonization• The first stage of microbial infection is colonization:

• Pathogens usually colonize host tissues that are in contact with the external environment.

• Sites of entry in human hosts include the urogenital tract, the digestive tract, the respiratory tract and the conjunctiva.

• Adherence to cell surfaces involves:

- Pili/fimbriae: primary mechanism in most gram negative cells.

- Teichoic acids: primary mechanism of gram positive cells.

- Adhesion: colonizing factor adhesions, pertussis toxin, and hemagglutinins.

- IgA proteses: cleaved Fc portion may coat bacteria and bind them to cellular Fc receptor.

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Definitions

•Carrier: person colonized by a potential pathogen without overt disease.•Bacteremia: bacteria in bloodstream without overt clinical signs.•Septicemia: bacteria in bloodstream (multiplying) with clinical symptoms.

Spores of fungi have a reproductive role.