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Dr Alvin Fox |
BACTERIOLOGY | IMMUNOLOGY | MYCOLOGY | PARASITOLOGY | VIROLOGY | |||
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BACTERIOLOGY - CHAPTER THIRTEEN STREPTOCOCCUS PNEUMONIAE
and Staphylococci |
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MICROBE RADIO |
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Reading: Murray, Third Edition Chapters 22 and 23 |
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KEYWORDS
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STREPTOCOCCUS PNEUMONIAE S. pneumoniae (figure 1) 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) (figure 2) Capsule This is highly prominent in virulent strains (figure 1c) 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.
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Figure 1B Scanning Electron Micrograph of Streptococcus pneumoniae. CDC/Dr. Richard Facklam
rrf2@cdc.gov |
Figure 1C Encapsulated Streptococcus pneumoniae
© Gloria J. Delisle and Lewis Tomalty, Queens University, Kingston,
Ontario and The MicrobeLibrary |
A
B
Figure 2 IIt is difficult to
distinguish normal alpha streptococci found in the mouth from the pathogenic
Streptococcus pneumoniae. Both are alpha-hemolytic on blood agar and so must be distinguished using the "P" disk
(optochin). S. pneumoniae (A) is sensitive while S. mitis (B) is resistant © Pat Johnson, Palm Beach Community College, Lake Worth Florida |
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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 (figure 3 and 4) (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 (figure 6). (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 (figure 7); 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
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 (figure 8) 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
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Figure 9 Two different species of Staphylococcus
growing on mannitol salt agar (MSA). MSA is selective because it
contains 7.5% salt–a high salt concentration that promotes the growth
of some organisms while discouraging the growth of others. MSA is
a differential medium because it contains the sugar mannitol and the pH
indicator phenol red. Organisms that can ferment mannitol produce
acid by-products, causing a color change. Phenol red is a cherry
red color above pH 8.5, yellow-red from pH 6.9 to 8.5, and bright yellow
at pH 6.9 or lower. Although both Staphylococcus epidermidis and
Staphylococcus aureus can tolerate the high salt content of MSA,
only S. aureus can ferment mannitol, causing the phenol red in
the medium to turn yellow.
© Margaret (Peg) Johnson, Mesa Community College, Mesa, Arizona and
The MicrobeLibrary
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MOVIE Cultures of Staphylococcus aureus (left) and Streptococcus pyogenes (right) were grown on blood agar plates for 16 h at 37 degrees. A colony from each plate was placed on a glass slide. A drop of 3% hydrogen peroxide was placed on both organisms. The catalase- producing organism catalyzes the breakdown of H2O2 to oxygen and water. O2 is released as bubbles. The catalase test is used to differentiate Staphylococcus sp. from Streptococcus sp. Staphylococcus sp. are positive for catalase production. Streptococcus sp. are negative for catalase production. © Neal R. Chamberlain, Department of Microbiology, Kirksville College of Osteopathic Medicine Kirksville, Missouri and The MicrobeLibrary |
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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.
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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.
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copyright 2006, The Board of Trustees of the University of South Carolina |
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