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Classification of Medically important cocci

gram positive- staphylococcus, streptococcus, enterococcus


gram negative- neisseria

Staphylococcus

Gram positive in clusters, 1 um in diameter


Facultative anaerobic


High salt tolerance- can grow on mannitol salt agar plate


Catalase positive- will bubble with hydrogen peroxide

Classification of staphylococcus

coagulase test- positive shows clotting when blood plasma added to bacteria in test tubes, no reaction for negative


positive- S. aureus


negative- S. epidermidis, S. saprophyticus

S. epidermis

adheres to prosthetics (implants, catheters) causing prosthesis infection

S. saprophyticus

causes acute cystitis, a UTI, in young women

S. aureus- virulence factors for attachment

teichoic acid on membrane (only found in Gram positive bacteria)

S. aureus virulence factors for evasion of host defence

* Coagulase, converting fibrinogen to fibrin
* Protein A, anti-phagocytic, reacts with the constant region (Fc) of IgG, meaning that the usual variable region which would allow the antibody to work on the bacteria cannot attach to it
* Catalase

S. aureus virulence factors for invasion

* Hyaluronidase, lipase, DNase, protease…

S. aureus virulence factors... toxins

* Epidermolytic toxins- causes lysis of skin cells, e.g. scalded skin syndrome
* Toxic shock syndrome toxin
* Enterotoxins- acts on GI tract causing food poisoning
* Panton-Valentine Leukocidin- acts on skin, dermonecrotic

Pyogenic diseases caused by S. aureus

Skin and soft tissue- e.g. folliculitis, which if S. aureus travels deeper into skin, worsens into furuncles


Osteomyelitis- inflammation of bone / bone marrow


Arthritis


Pneumonia


Infective endocarditis

Toxin mediated diseases caused by S. aureus

Scalded skin syndrome


Toxic shock syndrome


Food poisoning

Antibiotic sensitivity of S. aureus

Penicillin- yet now most S. aureus produce penicillinase, causing resistance


Methicillin and cloxacillin are alternatives, unaffected by penicillinase- yet growing MRSA


Vancomycin is another alternative, yet VRSA

Neisseria

Gram negative diplococci


Strictly aerobic


Two important species- N. meningitides (meningococcus) and N. gonorrhoeae (gonococcus)


Enzyme cytochrome oxidase found on membrane- turns oxidase reagent strips purple

Virulence factors of Neisseria

* Pili or fimbriae for attachment
* Capsule (in N. meningitidis), antiphagocytic
* Lipo-oligosaccharide is an endotoxin produced, which causes fevers

Neisseria gonorrhoeae clinical diseases

* Urethritis in men (can be confirmed by swabbing pus produced, and looking under microscope after Gram staining)
* Endocervicitis in women
* Pelvic inflammatory disease (uterus, fallopian tubes, ovaries)
* Proctitis (infection of the rectum due to anal sex)
* Pharyngitis (infection of the pharynx due to oral sex)
* Gonococcal ophthalmia neonatorum (infection of eyes of babies after passing through infected mother’s birth canal)
* Disseminated gonococcal infection
* Arthritis (uncommon)