Microbiology (W4)

Bacteria

Single-celled organisms constituting a large domain of prokaryotic microorganisms.

Archaea

Single-celled prokaryotic organisms that lack a defined nucleus.

Eukarya

Multi-cellular plants, animals and fungi.

Gram positive bacteria

Contain thick peptidoglycan layers that stain purple as it retains crystal violet dye.

Gram negative bacteria

Contain thin peptidoglycan layers and outer lipid membrane that stains pink/red.

Antibiotic resistance

Often more likely in Gram negative bacteria; they release endotoxins.

Examples of Gram positive cocci

Staphylococcus aureus, Streptococcus pneumoniae.

Examples of Gram positive rods

Bacillus anthracis, Clostridium tetani.

Examples of Gram negative cocci

Neisseria meningitidis, Neisseria gonorrhoeae.

Examples of Gram negative rods

Escherichia coli, Pseudomonas aeruginosa.

Testing order for identification

1. Catalase test

2. Hugh & Leifson test

3. Coagulase test

4. Novobiocin resistance test

Catalase test

Identifies organisms that produce enzyme catalase H2O2; bubbles = positive.

Hugh & Liefson test

Assesses glucose metabolism to see if bacteria is aerobic or anaerobic.

Coagulase test

Assesses presence of coagulase where fibrinogen forms fibrin.

Novobiocin resistance test

Determines if bacteria grow in the presence of novobiocin.

Virulence factors

Strategies produced by pathogens that enable them to colonise a host, evade immune defenses and cause disease.

Flagellum

A slender threadlike structure that enables bacteria to swim.

Fimbriae

An appendage for attachment found on many Gram-negative and some Gram-positive bacteria.

Pilus

A hair-like appendage for attachment found on the surface of many bacteria.

Plasmid

A small DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently.

Protein A

Specific to S. aureus and binds human IgG antibodies to provide 'disguise.'

Invasins

Interact with host cell-surface receptors to induce phagocytosis.

Effector proteins

Injected into host cells to hijack cellular processes.

Exotoxins

Secreted, soluble compounds produced by gram positive and negative cells.

Types of exotoxins

Cytotoxins

Neurotoxins

Enterotoxins

Cytotoxins

Diphtheria toxins, inhibits protein synthesis (extremely lethal).

Neurotoxins

Tetanus toxin, inhibits GABA or glycine release.

Enterotoxins

Secreted by enteric pathogens such as clostridium difficile; heat-stable proteins which produce pores leading to cell lysis.

Types of endotoxins

Lipopolysaccharides

Teichoic acid

Endotoxins

Found in cell walls and released upon bacterial death, causing massive inflammatory response when released into blood.

Lipopolysaccharide (LPS)

Found in all gram negative bacteria; Lipid A portion is highly immunoreactive.

Teichoic acid

Found in all gram positive bacteria.

Catalase test - Identification for gram positive cocci

Positive - Micrococcaneae

Negative - Streptoccocaceae

Hugh & Liefson test - Identification for gram positive cocci

Catalase test positive

Positive - Staphylococcus spp.

Negative - Micrococcus sp.

Coagulase test - Identification for gram positive cocci

Catalase + H&L test positive

Positive - Staph. aureus

Negative - Continue to novobiocin resistance test

Novobiocin resistance test - Identification for gram positive cocci

Catalase + H&L test postive and Coagulase test negative

Sensitive - Staph. epidermidis and other coag-ve Staph

Resistant - Staph. saprophyticus