Streptococcus constitute a large heterogeneous group of gram-positive bacteria that are very common parasites of man. Their name derives from their growth in liquid media as chains of globular bacteria. Classification of streptococci has been confusing, because a single species may cause a variety of diseases and because several kinds of streptococci can often be cultured from the same site of infection.
Classification of Streptococcus
The modern classification of streptococci is based upon Lancefields identification of the group-specific cell wall “C” carbohydrate antigens which are present in beta-hemolvtic streptococci and in some nonhemolytic species as well.
Group A Infections. The most important human streptococcal pathogen. Streptococcus pyogenes, can be identified as containing group A carbohydrate with specific rabbit antiserum to the carbohydrate cell wall antigens, and thus can be distinguished from other beta-hemolytic streptococci which are also frequently isolated from the respiratory tract of man. Diseases caused by group A streptococci will be considered in detail in Ch. 185 to 191 and in Ch. 610. Streptococci belonging to other sero-groups that are of importance to man will be mentioned briefly here.
The traditional classification of streptococci by their properties of hemolysis on the surface of blood agar plates is clinically useful. It helps to distinguish hemolytic streptococci, to which the more virulent species belong, from the alpha-hemolytic Streptococcus viridans (“green” streptococci), the predominant normal flora of the upper respiratory tract, and the nonhemolytic streptococci which include the enterococci, a common inhabitant of the gastrointestinal tract.
Beta-hemolytic streptococci are those that produce a completely clear zone around the colony as a result of the formation of either extracellular hemolysin, streptolysin O and streptolysin S, or both. Classification of streptococci based on the type of hemolysis they produce, however, is unsatisfactory for many reasons. Many species belonging to specific serologic groups may be nonhemolytic, such as group I). On the other hand, some enterococci belonging to the usually nonhemolytic group D may produce beta hemolysis, or some apparently nonhemolytic colonies may become alpha hemolytic upon prolonged incubation. Beta hemolysis is therefore primarily useful in the first step of the identification of group A organisms which are. with only rare exceptions, consistently hemolytic.
Group B Infections.
The group B organisms arc classically represented bv the species Streptococcus agalactiae. They were well known for many years as the cause of bovine mastitis but produced human disease rarely. Until recently, little attention was paid to group B organisms in man, although it was known that they could colonize the vagina and genitourinary tract and were occasionally found in the upper respiratory tract as well.
IN fact, between 5 and 10 per cent of hemolytic streptococci isolated from man were identified in many studies as group B. In the past decade, however, group B streptococci have been reported with increasing frequency as a cause of perinatal infections. In the preantibiotic era, group A streptococci were the common agent of puerperal fever. Since the early 1960’s, however, group B streptococci have emerged as a major cause of puerperal infection and as a cause of neonatal sepsis, with or without meningitis.
The frequency with which group B streptococci are recovered from the female genital tract (and the male genitourinary tract as well) appears to be increasing, and cervicovaginal carrier rates in normal pregnant women vary from 3 to 6 per cent. In some studies, approximately one third of infants born to such carriers have contracted serious group B infections. Group B organisms also seem to he causing bacteremic complications of pyelonephritis in both men and women more often than heretofore, especially in diabetics and other compromised hosts. These infections have remained susceptible to penicillin so far.
Group D Infections.
This group includes Streptococcus faecal is, often referred to as enterococcus because of the frequency with which it is found in the human gastrointestinal tract. Enterococci may also be cultured from the oropharynx and, although usually nonhemolytic, some strains can produce beta or alpha hemolysis. S. faecal is is an exceptionally hardy organism which can resist heat (62°C for 30 minutes), grow well at room temperature, multiply in hypertonic media (6.5 per cent sodium chloride), and grow in the presence of 0.05 per cent sodium azide.
Enterococci are often isolated from the blood in bacterial endocarditis and from the urine in urinary tract obstruction. The precise role of these organisms in the pathogenesis of pyelonephritis has not been fully evaluated. They can be associated with suppurative abdominal lesions, especially after bowel surgery.
The failure of some laboratories to distinguish the enterococci from other streptococci often leads to inappropriate therapy in serious infections. These organisms may also be recovered from the respiratory tract and may be regarded mistakenly as penicillin-resistant group A streptococci. This error occurs most often during the post-treatment follow-up study of cases of group A streptococcal sore throat and may lead to needless over-treatment in an attempt to eradicate the organisms from the pharynx. It cannot be emphasized too strongly that enterococci do not cause pharyngitis and tonsillitis.
Streptococcus of Other Groups.
Strains of groups C, E, G, H, K, and 0 are isolated from the respiratory tract of man but are of little clinical significance. “Human” group C strains may occasionally cause illnesses that resemble those of group A, but such infections are relatively rare. Moreover, as far as is known, only group A organisms cause rheumatic fever. Group G may occasionally cause mild infections, but these are relatively infrequent and are uncomplicated. Groups A, C, and G all elaborate in vivo antigenically similar streptolysin 0, streptokinase, hyaluronidase, and erythrogenic toxin.
Alpha-Hemolytic Streptococci (“Green” Streptococcus ).
The alpha-hemolytic streptococci are often referred to collectively as the viridans group. They have never been satisfactorily classified. Their colonies are surrounded by a narrow zone of incompletely hemolyzed red cells (some red cells are spared for unknown reasons!. Green discoloration of the colonies occurs owing to the formation of an unidentified reductant of hemoglobin. The degree of “greening” varies with the animal source of blood and is best brought out by sheep red blood cells which differentiate most clearly beta from alpha hemolysis.
Streptococcus viridans is the predominant normal flora of the upper respiratory tract and is generally non-pathogenic except as a cause of subacute bacterial endocarditis. Streptococcus salivarius is one of the most commonly encountered of these species.
The term nonhemolytic Streptococcus is confusing because it is often used to include any Streptococcus that is not beta hemolytic, and because many nonhemolytic species (including the common Streptococcus faecalis) possess the same group-specific cell wall antigens as certain hemolytic strains. The organisms of the nonhemolytic group are generally of low pathogenicity for man and, like alpha-hemolytic streptococci, are of concern to physicians primarily as causative agents of subacute bacterial endocarditis. They may multiply and cause inflammation in traumatized or diseased structures and wounds and in obstructed sinuses, bronchi, and urinary and biliary tracts.
The varieties of streptococci considered above are facultative anaerobes. Microaero-philic streptococci which are obligate anaerobes do exist, however, and cause human disease. They are usually nonhemolytic and have not been systematically classified. These organisms are found in the mouth, bowel, and female genital tract. Their virulence is low and they tend to multiply in necrotic or frankly gangrenous lesions, producing a foul odor such as may be noted in lung abscesses or intrauterine infections. They may cause very extensive narcotizing wound infections. Although most anaerobic streptococci are susceptible to antimicrobial drugs, the lesions in which they are found usually require adequate surgical drainage as well as chemotherapy.