Streptococcus Overview

Gram positive cocci in pairs/chains à can be either Strep or Enterococcus
Classification is mainly determined by Hemolytic properties on blood agar plates

A. Alpha-Hemolytic Strep – partial hemolysis, giving a greenish color
1. Strep pneumo – distinguished from strep viridians by the polysaccharide capsule and optochin sensitivity. Frequently colonizes the nasopharynx, and causes many diseases that we are quite familiar with: Community-acquired pneumonia, meningitis, otitis media, sinusitis, bacteremia
2. Strep viridians – characterized by the lack of polysaccharide capsule and optochin resistance. Are part of the normal oral and GI flora. They are important clinically mainly due to their ability to enter the bloodstream and cause subacute bacterial endocarditis in patients with damaged heart valves.

Strep mitis
Strep mutans – causes dental caries
Strep oralis
Strep sanguinis
S.milleri/anginosus group (S.constellatus, S.intermedius, S.anginosus) – can be beta or alpha-hemolytic. When beta-hemolytic, can be Lancefield A, C, F, and G. Famous for their ability to cause abscesses: dental, liver, brain, lung, perinephric, subcutaneous abscesses, and more.

B. Beta-Hemolytic Strep – complete hemolysis
Further classified by their Lancefield groups, which are the polysaccharide antigens on the cell surface.
Note that not all strep species fit cleanly into one Lancefield group (or hemolysis pattern for that matter, like the Strep milleri group); for example, Strep dysgalactiae can be Group C or Group G.

1. Group A Strep (Strep pyogenes) – causes pharyngitis, acute rheumatic fever, skin/soft tissue infections (cellulitis, scarlet fever, necrotizing fasciitis), toxic shock syndrome, acute glomerulonephritis

2. Group B Strep (Strep agalactiae) – causes disease in neonates (neonatal sepsis, meningitis), pregnant women (UTIs), and nonpregnant adults, especially predisposed by diabetes and other chronic diseases. Causes SSTI, bacteremia, UTI, PNA, endocarditis, osteomyelitis, and more.

(Click here for more information on Group B Strep.)

3. Group D Strep – some of these species were reclassified as Enterococcus in 1984. Remaining species include Strep bovis, famous for its association with colon cancer (~25% of pts with Strep bovis bacteremia have an underlying colon cancer), and Strep equinis.

4. Group C and Group G strep – less extensively studied but becoming increasingly recognized as an important pathogen, especially in Japan (for unclear reasons). Colonize the skin flora, oropharynx, and GI/GU tracts. Appear to cause infections similar to Group A strep as they share many of the same virulence factors.
Broken down into Large colony and Small colony.
a. Large colony Group C/G Strep – most important is Strep dysgalactiae subspecies equisimilis.

As mentioned, causes infections similar to GAS (pharyngitis, cellulitis/SSTI including necrotizing fasciitis and Toxic Shock Syndrome), septic arthritis, bacteremia, endocarditis, pneumonia, and more.
When bacteremia is present, the most common source is cellulitis (~60% of cases) although many times no focus is identified.
Risk factors including being old, males (>2:1 vs females), diabetes, chronic cardiac disease, CKD, cirrhosis, alcoholism, immunocompromised state, and surgical procedures.
Of note, there are case reports of vertebral osteomyelitis caused by Group G strep, as in our patient.
Group C/G strep are uniformly penicillin sensitive.

b. Small colony Group C/G strep – refers to the Strep anginosis/miller group (see above in the Alpha-hemolytic strep section).

C. Gamma-Hemolytic Strep - no hemolysis. Rarely causes disease in humans so not clinically important.

(Chanu Rhee MD, 4/1/11)