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Gram Positive Cocci

Background & Introduction:

Gram-positive cocci are often isolated from clinical samples. It is important to be able to distinguish these organisms from one another because while some are normal flora, others can be pathogenic.  Below are some commonly encounter gram-positive cocci.

STAPHYLOCOCCUS:

  1. Gram-positive cocci, arranged in grape-like clusters

  2. Catalase positive

  3. High salt tolerance

  4. Facultative anaerobes

  5. Non-motile

Three common species are...

  • Staphylococcus aureus

  • Staphylococcus epidermidis

  • Staphylococcus saprophyticus

Staphylococcus spp. are normal flora of skin and mucous membranes, however they can cause disease when introduced into normally sterile sites in the body. Staphylococcus aureus is the most pathogenic species in the group. The virulence of Staphylococcus aureus is due to a number of enzymes or toxins that some strains can produce.

For example:

  1. Coagulase: clots plasma

  2. Hemolysins: damages RBC

  3. Leukocidins: kill WBC

  4. Toxins: necrosis of the skin

  5. Enterotoxin: food poisoning

The diseases associated with pathogenic Staphylococcus aureus infections include:

  1. boils

  2. abcesses

  3. wound infections

  4. bacterial pneumonia

  5. food poisoning

  6. toxic shock syndrome

MICROCOCCUS:

  1. Gram-positive cocci arranged as single cells, pairs or irregular clusters

  2. Catalase positive

  3. High salt tolerance

  4. Strict aerobes

  5. Non-motile

  6. Form pigmented colonies

There are nine species in this genus. The species that we work with in lab is Micrococcus luteus. Most Micrococcus spp. colonize human skin as normal flora but some can cause opportunistic infections.

 

STREPTOCOCCUS:

  1. Gram-positive cocci, arranged in pairs or chains

  2. Catalase negative

  3. Many are hemolytic (attack red blood cells)

  4. Anaerobic, but tolerate the presence of oxygen (aerotolerant)

  5. Non-motile

  6. Often assembled according to serological properties (groupings A through H and K through V).

A number of relevant species include...

  • Streptococcus pyogenes: strep throat, scarlet fever, rheumatic fever, etc. (Group A).

  • Streptococcus pneumoniae: pneumonia

  • Streptococcus agalactiae: normal flora, infections of neonates and infants (Group B).

  • Streptococcus mitisStreptococcus salivarius: normal flora of the mouth

  • Streptococcus mutans: dental caries

Streptococci are responsible for a large variety of infectious diseases. The pathogenesis of Streptococcus spp. is due to various enzymes and toxins they can produce.

For example:

  1. Erythrogenic toxin: scarlet fever rash

  2. Streptolysins: strep throat

  3. Hyaluronidase: breaks down connective tissue

  4. Streptokinase: digests fibrin in plasma

 

ENTEROCOCCUS:

  1. Gram-positive cocci arranged in pairs or short chains

  2. Catalase negative

  3. Facultatively anaerobic

  4. Hydrolyze esculin in the presence of bile

  5. Fairly halotolerant

  6. Non-motile

  7. Localized to the enteric region.

Relevant species:

  • Enterococcus faecalis

  • Enterococcus faecium

Enterococci have a limited potential to cause disease as they lack defense systems against phagocytic cells. However, enterococci commonly cause nosocomial urinary tract infections, bacteremia and wound infections.

Because StreptococcusEnterococcus, Micrococcus, and Staphylococcus spp. are all Gram-positive cocci, and because the four genera are comprised of both normal flora and pathogenic species, it is essential to distinguish between these organisms in the clinical microbiology laboratory. In the next couple of labs, the key biochemical tests used to identify the species of these genera will be introduced.

Background information for the above discussion was obtained from Medical Microbiology by Murray et. al.

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