Streptococcus pneumoniae & Staphylococci

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STREPTOCOCCUS PNEUMONIAE

S. pneumoniae is a leading cause of pneumonia in all ages (particularly the young and old), often after "damage" to the upper respiratory tract (e.g. following viral infection). It also causes middle ear infections (otitis media). The organism often spreads causing bacteremia and meningitis. S. pneumoniae is α hemolytic and there is no group antigen.

Direct Gram staining or detection of capsular antigen in sputum can be diagnostic. The organism grows well on sheep blood agar.

Autolysin

Pneumococci are identified by solubility in bile. An autolysin (peptidoglycan-degrading enzyme) is released by bile from the cell membrane and binds to a choline-containing teichoic acid attached to the peptidoglycan. The autolysin then digests the bacterial cell wall resulting in lysis of the cell. If the cells are grown in ethanolamine instead of choline, ethanolamine is incorporated into the teichoic acid. The autolysin then cannot lyse the cell wall. Understanding how the autolysin works has led to the suggestion that antibiotics (including penicillin) work together with the autolysin in killing of pneumococci in vivo.

The organisms are also identified by susceptibility to optochin (ethyl hydrocupreine)

Capsule

This is highly prominent in virulent strains and its carbohydrate antigens vary greatly in structure among strains. The capsule is anti-phagocytic and immunization is primarily against the capsule. Capsular vaccines are available for susceptible individuals; immunity is serotype-specific. Using appropriate type-specific antisera, the capsule on isolated bacteria can be "fixed" and becomes visible microscopically (the Quellung reaction) which is useful in microbial identification.

The organism also produces pneumolysin that degrades red blood cells under anaerobic conditions (observed as α hemolysis).

Complement activation by teichoic acid may explain the attraction of large numbers of inflammatory cells to the focal site of infection.

Most strains of S. pneumoniae are susceptible to penicillin. However, resistance is quite common

STAPHYLOCOCCI

Facultative anaerobes, Gram positive, occur in grape like-clusters and are catalase positive. Major components of the normal flora of skin and nose.

Staphylococcus aureus

(i) One of the commoner causes of opportunistic infections in the hospital and community; including pneumonia, osteomyelitis, septic arthritis, bacteremia, endocarditis, abscesses/boils and other skin infections

(ii) Food poisoning. The food becomes contaminated with the organism from human contact, grows and produces enterotoxin. The organism does not "infect" on ingestion of food. Thus, onset and recovery both occur within a few hours. Vomiting, nausea, diarrhea and abdominal pain are seen.

(iii) Healthy people: boils.

(iv) Toxic shock syndrome particularly after tampon use; includes fever, rash, desquamation, vomiting, diarrhea; toxic shock toxin involved. The organism does not disseminate. However, the toxin does and is responsible for the clinical features.

(v) Exfoliative toxin causes scalded skin syndrome in babies.

Identification

beta-hemolytic on sheep blood agar
Mannitol fermentation
Golden pigmented (aureus)- often
Coagulase-positive

In reference laboratories phage-typing is used.

As noted above, S. aureus causes a number of different disease entities associated with production of certain exotoxins. In addition to these "disease-specific" exotoxins, other cell lytic exotoxins (alpha, beta [sphingomyelinase C], gamma and delta toxins and leucocidins) may be produced. Also some tissue-degrading enzymes may be involved in spreading (e.g. lipase and hyaluronidase).

Free protein A binds to immunoglobulin and complement, blocking Fc and complement receptors and is thus anti-phagocytic.

Staphylococcus epidermidis

Staphylococcus epidermidis is a less common cause of opportunistic infections than S. aureus, but is still significant. It is a mediator of nosocomial infections (e.g. catheters, shunts, surgery [e.g. heart valves]). It is a major component of the skin flora and thus commonly a contaminant of cultures.

Identification

Non-hemolytic on growth on sheep blood agar
Does not ferment mannitol (figure 9)
Non-pigmented
Coagulase-negative.

Staphylococcus saprophyticus

This organism is a significant cause of urinary tract infections. It is also coagulase-negative and is not usually differentiated from S. epidermidis clinically.

Antibiotic therapy

Staphylococci (including both coagulase positive and coagulase negative organisms) can produce a phage-coded penicillinase that degrades beta lactam antibiotics. Some strains also have modified penicillin binding proteins. Thus beta lactam antibiotics (including methicillin) are often ineffective. Vancomycin is thus the drug of choice.