Streptococci

Streptococci are gram positive cocci arranged in pairs or chains. In Greek streptos means chain, coccos means a grain or berry. This arrangement of ‘chains’ occurs beause they divide in a single plane unlike Staphylococci which divide in multiple planes and appear in clusters. They do not possess the enzyme catalase and are facultative anaerobes responsible for a wide variety of infections in humans. They are fastidious in their growth and often require supplementation with blood. Some Streptococci require pyridoxyl for development and are called nutritionally variant Streptococci (e.g Abiotrophia species, Granulicatella species)

Classification

Classification of Streptococci is based on their action on blood agar and the different antigens present in their cell wall. Accordingly, based on the type of hemolysis they produce on blood agar, three types are seen.  They are as follows:

• β hemolytic : Streptococci which are capable of complete destruction of red blood cells (RBCs) resulting in a complete clearing of the blood agar medium are called β hemolytic Streptococci. e.g Streptococci pyogenes
• α hemolytic : Some Streptococci reduce the hemoglobin in the RBCs  producing a green pigmentation around the colonies. e.g S.viridans
• γ hemolytic : Streptococci which do not result in any change in the RBCs are called γ hemolytic.

Rebecca Lancefield, an American Microbiologist, pioneered the serological classification of β hemolytic streptococci based on the major cell wall polysaccharide as serogroups A, B, C, D etc.

Streptococcus species of clinical importance

• Pyogenic group:
  1. Streptococci pyogenes
  2. Streptococci agalactiae
  3. Streptococci equisimilis which are both human and animal pathogens.

Streptococci pyogenes is a primary pathogen in this group causing invasive and non-invasive infections. Non-invasive infections include but are not limited to, pharyngitis, scarlet fever and suppurative skin infections such as pyodermas or impetigo. Invasive soft tissue infections include necrotizing fascitis, myositis, toxic shock syndrome and bacteraemia. Rheumatic fever and acute glomerulonephritis are the serious non-suppurative sequelae of Streptococci pyogenes infections. Streptococci pyogenes is a delicate organism and is easily destroyed at 54ºC in 30 minutes. It is rapidly inactivated by antiseptics. Susceptibility to bacitracin is a convenient method of identifying S.pyogenes from other beta hemolytic streptococci. Streptococci agalactiae is an important pathogen in neonatal sepsis. It colonizes the maternal genital tract and is swallowed by the neonate during labor. The CAMP (Christie-Atkins-Munch-Peterson) test is a useful test to identify Streptococci agalactiae from clinical specimens.

• Mitis group: This group includes the commensals of the human oral cavity however Streptococci pneumoniae is an important pathogen causing lower respiratory tract infections. It is a capsulated lanceolate shaped diplococcus. Streptococci pneumoniae is the most frequent cause of pneumonia with an annual incidence of 1 to 3 per 1000 population. The rates may differ especially in high risk groups such as children, alcoholics and immunocompromised patients. Meningitis is the most serious of pneumococcal infections. Pneumococcal meningitis occurs in all age groups and without antimicrobial therapy it is invariably fatal. Even with antimicrobial therapy, the mortality can be as high as 25% .Streptococci mitis, a commensal of the oral cavity, is isolated frequently from blood cultures and is of significance due to the emergence of resistance to penicillin and other betalactam agents.
• Anginosus & Salivarius group: They are also a part of the flora of the oral cavity and the pharynx. The Anginosus group is involved in intra-abdominal infections such as hepatic, subphrenic and pancreatic abscess.
• Bovis group: these reside in the human colon. The organisms included in this group are Streptococci bovis, Streptococci suis, Streptococci alactolyticus etc. Streptococci bovis causes bacteremias, meningitis and native and prosthetic valve endocarditis. Invasive disease has often been seen in patients suffering from colon cancers. An association between disseminated Streptococci bovis infections and ulcerative colitis and chronic radiation enterocolitis has also been found. Streptococci suis is an critical pathogen in swine. Human infections occur due to consumption of contaminated pork or percutaneously via cuts, scratches or abrasions on the skin while dealing with the infected animals. Post-infection, the prodromal phase is an ‘influenza-like’ illness followed by the rapid onset of bacteremia and meningitis. The most common sequelae following meningitis is cochlear vestibular involvement leading to ataxia and giddiness. Facial nerve involvement may result in unilateral or bilateral deafness.
• The mutans group is a commensal of the tooth surfaces in man and animals. They have been incriminated in the formation of dental caries.

Vaccines against Streptococci pneumoniae

Streptococci pneumoniae is capable of causing serious invasive infections especially in patients who are at high risk. Vaccine prophylaxis assists in prevention of severe disease as well as reducing the morbidity and mortality related to invasive pneumococcal infections. The pneumococcal capsular polysaccharide shows antigenic diversity. Accordingly, there are more than 90 serotypes of S.pneumoniae present. Immunity to pneumococcal infections is type specific and associated with the antibody to the capsular polysaccharide. Two vaccines are available to prevent invasive pneumococcal infections.

1. Polyvalent Polysaccharide vaccine (PPV):  This vaccine contains the 23 most prevalent capsular antigens and offers about 80% to 90% protection. It must be given to individuals who are at high risk, such as people over 65 years of age, those with sickle cell disease, celiac disease, chronic liver disease or chronic renal disease. It is not recommended for children under 2 years of age, patients on immunosuppressive therapy and those with lymphoreticular malignancies. It is administered as a subcutaneous or an intramuscular injection. In patients who are undergoing an elective spleenectomy, the vaccine is recommended at least 2 weeks prior to surgery.
2. Pneumococcal conjugate Vaccine 7 (PCV): PCV7 is prepared using an inert Diphtheria toxin carrier protein (CRM197) which is covalently linked to the seven pneumococcal capsular serotypes primarily 4, 6B, 9V, 14, 18C, 19F and 23F. It is administered as an intramuscular injection to infants and children starting at 2, 4 and 6 months followed by a booster dose at 12 to 15 months. The Advisory Committee on Immunization Practices (ACIP) does not recommend the use of PCV in children 5 years of age or older. However, the vaccine is licensed for use in children up to 9 years of age. Therefore, in special situations, the vaccine can be administered to older children who are at a high risk of invasive infection such as anatomic or functional asplenia or HIV positive children.

Both PPV and PCV can be administered to the same individual. In such a scenario, PCV must be administered first, followed by PPV after an 8 week interval.