Microbiology: Gram Positive Cocci

The Genus Staphylococcus

• Gram-positive in reaction
• Arranged in grape-like irregular clusters
• Comprises over 30 species, with 3 main species of clinical importance: S. aureus, S. epidermidis, and S. saprophyticus

General Properties

• Grow on many types of media
• Some species are part of the normal human flora (skin and mucous membranes), while others cause infections
• Pathogenic staphylococci can hemolyze blood, coagulate plasma, and produce extracellular enzymes and toxins

Identification

• Can grow on most bacteriological media under aerobic and microaerophilic conditions
• Grow rapidly at 35-37°C
• Colonies on solid media are round, smooth, and raised
• S. aureus forms gray to deep golden yellow colonies, while S. epidermidis produces gray to white colonies

Growth Characteristics

• Produce catalase, which converts H2O2 into H2O and O2
• Ferment carbohydrates, releasing lactic acid but not gas
• Produce many extracellular substances
• Resistant to drying, heat (50°C for 30 minutes), and 9% NaCl
• Variable sensitivity to many antibiotics (ABCs)
• Some species produce β-lactamase due to plasmid genes, making them resistant to ampicillin, tetracycline, and aminoglycosides

Antigenic Structure

• Contain antigenic polysaccharides, peptidoglycan, teichoic acids, proteins, and other substances like capsules and coagulase (in S. aureus)

Toxins and Enzymes

• Enzymes:
• Catalase, which converts H2O2 into H2O and O2, differentiating staphylococci from streptococci
• Coagulase, produced by S. aureus, which clots plasma, deposits fibrin on the surface of staphylococci, inhibits phagocytosis, and is associated with pathogenicity

General Characteristics of Neisseria

• Non-motile, gram-negative, intracellular diplococci that ferment carbohydrates, producing acid but not gas
• Kidney-shaped with adjacent concave sides

Optimal Growth Conditions

• Grow best on complex media under aerobic conditions containing 5% CO2
• Require heated blood, hemin, animal protein, and other supplements for growth

Medical Importance

• Two main species of medical importance: N. meningitidis and N. gonorrhoeae
• Neisseria species are sensitive to drying, sunlight, moist heat, and disinfectants

Neisseria gonorrhoeae

General Characteristics

• Obligate parasite of the human urogenital tract
• Has no polysaccharide capsule but has multiple serotypes based on pilus protein antigenicity
• More than 100 serotypes are known due to antigenic variation in gonococcal pili

Virulence Factors

• Pili: mediate initial attachment to non-ciliated human cells, resist phagocytosis
• Por protein (Protein I): forms pores on the surface of bacteria, promotes intracellular survival by preventing phagolysosome fusion in neutrophils
• Opa (Protein II): important for firm attachment and invasion of host cells
• Lipooligosaccharide (LOS): responsible for symptoms/toxicity of gonorrhea, triggers intense inflammatory response, and activates complement

Pathogenesis and Clinical Manifestations

• Transmitted by sexual contact
• Gonococci attack mucous membranes of the GUT, eye, rectum, and throat
• In males, causes urethritis with yellow, creamy pus and painful urination
• In females, primary infection is in the endo-cervix, extending to the cervix and vagina, causing mucopurulent discharge
• Can cause gonococcal ophthalmia neonatorum in newborns

Laboratory Diagnosis

• Specimen: Urethral swab