| Streptococcus pneumoniae
Medical Microbiology, MBIM
650/720 Lecture: 39
Streptococcus pneumoniae in spinal fluid. FA stain
(digitally colorized). CDC/Dr. M.S. Mitchell
Scanning Electron Micrograph of Streptococcus pneumoniae.
CDC/Dr. Richard Facklam email@example.com
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 a 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.
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
are also identified by susceptibility to optochin (ethyl hydrocupreine)
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
reaction) which is useful in microbial identification.
The organism also
produces pneumolysin that degrades red blood cells under anaerobic
conditions (observed as a hemolysis).
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
anaerobes, Gram positive, occur in grape like-clusters and are catalase
positive. Major components of the normal flora of skin and nose.
Staphylococcus aureus - MRSA resistant coccoid prokaryote
(dividing); causes food poisoning, toxic shock syndrome and
skin and wound infections (scalded skin syndrome, scarlet
fever, erysipelas, impetigo, etc.) © Dr
Dennis Kunkel, University of Hawaii. Used with permission
Staphylococcus aureus (Gram-positive) ©
Linda M Stannard, 1996. Used with permission
Staphyllococcus aureus - Acridine-orange leucocyte cytospin
test © Bristol Biomedical Image Archive. Used
(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.
Staphylococcal Infection: Impetigo © Bristol Biomedical
Image Archive. Used with permission
(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.
(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.
Box of Rely tampons. Associated with outbreak of toxic shock syndrome.
toxin causes scalded skin syndrome in babies.
sheep blood agar
Golden pigmented (aureus)- often
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.
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
growth on sheep blood agar
Does not ferment mannitol
A significant cause
of urinary tract infections. Also coagulase-negative and not usually
differentiated from S. epidermidis clinically.
(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 ß lactam antibiotics (including methicillin) are
often ineffective. Vancomycin is thus the drug of choice.