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Streptococci and enterococci 

Streptococci and enterococci

Chapter:
Streptococci and enterococci
Author(s):

Dennis L. Stevens

DOI:
10.1093/med/9780199204854.003.070602_update_001

Update:

Changes in epidemiology of invasive group B streptococcal disease.

Risk factors and prevention strategies for early-onset neonatal group B streptococcal disease.

Additions to Further reading.

Updated on 31 May 2012. The previous version of this content can be found here.
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date: 24 April 2017

The streptococci are a diverse group of Gram-positive pathogenic cocci that cause clinical disease in humans and domestic animals. They are traditionally classified on the basis of serological reactions, particularly Lancefield grouping based on cell-wall carbohydrates, and haemolytic activity on blood agar. Six groups can be defined by genetic analysis: pyogenic streptococci, milleri or anginosus group, mitis group, salivarius group, mutans group, and bovis group.

Group A streptococci (S. pyogenes)

Group A streptococci are carried, usually in the nose or throat, by 5 to 20% of children and 0.5% of adults. More than any other human pathogen, group A streptococci cause a wide variety of infections ranging from pharyngitis, erysipelas, cellulitis, and necrotizing fasciitis to the postinfectious sequelae: rheumatic fever and poststreptococcal glomerulonephritis. These microbes continue to evolve, as evidenced by over 150 different genetic types and the emergence of novel infections such as streptococcal toxic shock syndrome.

Group A streptococci are easy to culture in the laboratory from appropriate samples; diagnosis can also be made by detection of the group A antigen or confirmed serologically. All strains remain sensitive to penicillin, which is the antibiotic of choice, with erythromycin usually given to those who are penicillin allergic, although epidemics of pharyngitis caused by erythromycin resistant strains have been widely reported. Genetic differences and the presence of multiple virulence factors have frustrated efforts to develop effective vaccines.

Group B streptococci (S. agalactiae)

Group B streptococci are carried in the throat by 5 to 10% of adults, as well as in the urethra, vagina, perineum, and anorectum. They cause a variety of infections: (1) neonatal infection—including bacteraemia and meningitis; screening for vaginal carriage during the third trimester of pregnancy and intrapartum treatment with intravenous penicillin has reduced the incidence of early onset neonatal disease; (2) postpartum infection—puerperal infection usually manifests as endometritis with fever and uterine tenderness, occurring within 24 to 48 h of delivery or abortion; also (3) skin and soft tissue infections, urinary tract infections, and bacteraemias (especially in patients with diabetes mellitus, malignancy, HIV infection, and chronic renal or liver disease).

Group B streptococci are readily isolated from any clinical specimen in the laboratory, and detection of group B antigen in body fluids by latex particle agglutination enables rapid diagnosis. They are sensitive to penicillin (the antibiotic of choice), erythromycin and cephalosporins. The polysaccharide capsule of group B streptococcus is a major virulence factor, with at least six different serotypes identified: experimental immunization using the polysaccharide provides type specific protection, but no such vaccine has yet been developed for human use.

Other groups of streptococci

Groups C and G—produce infections that are similar to those caused by group A streptococci but tend to be less virulent. They are important causes of cellulitis, particularly recurrent cellulitis associated with saphenous vein donor site infections in patients with coronary artery by-pass surgery.

Streptococcus milleri or Streptococcus anginosus group—includes S. constellatus, S. intermedius, and S. anginosus. These are found in the normal flora of the upper respiratory tract, gastrointestinal tract and genital tract; commonly isolated from a range of pyogenic infections (e.g. dental or other abscesses), sometimes in pure culture, but often with other organisms, particularly anaerobes.

Streptococcus mitis, Streptococcus salivarius, and Streptococcus mutans groups of streptococci (oral/viridans streptococci)—these include S. pneumoniae (see Chapter 7.6.3) and those oral streptococci that are the commonest causes of infective endocarditis of oral or dental origin. Occasionally cause bacteraemia in neutropenic patients, particularly those who have received prophylaxis with fluoquinolones such as ciprofloxacin.

Streptococcus bovis group—a gastrointestinal commensal; most patients with S. bovis bacteraemia will have endocarditis in association with colonic pathology or cirrhosis of the liver.

Streptococcus suis—an occupational cause of septicaemia, meningitis, septic arthritis, pneumonia, and endophthalmitis among those working with pigs and pork in South-East Asia.

Enterococci

Part of the normal gut flora of humans and animals, these are an increasingly important cause of nosocomial infection and colonization, possibly the result of the large-scale use of antibiotics such as cephalosporins and quinolones to which they are inherently resistant. Enterococcus faecium and E. faecalis have also become vancomycin resistant, a characteristic dramatically increasing treatment failures, although they remain sensitive (at the time of writing) to linezolid, an oxazolidinone antimicrobial.

Acknowledgement: The author of the present chapter and the editors acknowledge the inclusion of much material from the chapter in the previous edition by Professor S K Eykyn.

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