Chapter 25: Streptococci and Enterococci

Overview

Members of the genus Streptococcus and enterococci are all Gram-positive cocci that grow in pairs or short to long chains (Figure 25–1) in contrast to the clusters seen with staphylococci. Furthermore, streptococci and enterococci are catalase-negative, whereas staphylococci are catalase-positive. Streptococci and enterococci are classified principally based on their patterns of hemolysis. Streptococci showing β-hemolysis (Figure 25–2) are grouped according to the carbohydrate antigens extracted from their cell walls. Groups A and B are the leading pyogenic pathogens of the streptococci having β-hemolysis and cause diverse clinical syndromes. Group A streptococci are the cause of “strep throat,” an acute inflammation of the pharynx and tonsils that includes fever and painful swallowing. Skin and soft tissue infections range from the tiny skin pustules called impetigo to a severe toxic and invasive disease that can be fatal in a matter of days. In addition to acute infections, group A streptococci are responsible for inflammatory diseases that are not direct infections but result from an immune response to streptococcal antigens that cause injury to host tissues. Acute rheumatic fever (ARF) is a clinical entity characterized by prolonged febrile inflammation of connective tissues, which can recur after each subsequent attack of streptococcal pharyngitis. Repeated episodes cause permanent scarring of the heart valves. Acute glomerulonephritis is an insidious disease with hypertension, hematuria, proteinuria, and edema due to inflammation of the renal glomerulus.

Group B streptococci (GBS) are harbored in the human gut but may colonize the urethra and vagina. If present in mothers at the time of parturition, their newborns are at risk for severe invasive disease. Insidious initially with fever, lethargy, poor feeding, and respiratory distress, the etiologic diagnosis is disclosed only by isolation of GBS from blood or cerebrospinal fluid.

The α-hemolytic streptococci include Streptococcus pneumoniae and the viridans group streptococci. The most common form of infection with S pneumoniae is pneumonia, which begins with fever and a shaking chill followed by signs that localize the disease to the lung. These include difficulty breathing and cough with production of purulent sputum, sometimes containing blood. The pneumonia typically fills part or all of a lobe of the lung with inflammatory cells, and the bacteria may spread to the bloodstream and thus to other organs. The most important of the latter is the central nervous system, where seeding with pneumococci leads to acute purulent meningitis. Pneumococci are also a leading cause of otitis media in the early childhood.

Viridans group streptococci are a heterogeneous group of α-hemolytic streptococci that usually are commensal flora of the pharynx and gut but may cause invasive disease such as abscesses or bacterial endocarditis. The S anginosus group in particular causes abscesses, notably in the liver and brain. Pyridoxal-requiring streptococci (Granulicatella and Abiotrophia) are prone to cause endocarditis as is the ...