Viridans Streptococci

Viridans Streptococci
The viridans streptococci include S mitis, S mutans, S salivarius, S sanguis, and others. Typically they are alpha -hemolytic, but they may be nonhemolytic. Their growth is not inhibited by Optochin, and colonies are not soluble in bile (deoxycholate). The viridans streptococci are the most prevalent members of the normal flora of the upper respiratory tract and are important for the healthy state of the mucous membranes .They may reach the bloodstream as a result of trauma and are a principal cause of endocarditis on abnormal heart valves. Some viridans streptococci (eg, S mutans) synthesize large polysaccharides such as dextrans or levans from sucrose and contribute importantly to the genesis of dental caries.
In the course of bacteremia, viridans streptococci, pneumococci, or enterococci may settle on normal or previously deformed heart valves, producing acute endocarditis. Subacute endocarditis .

Nutritionally Variant Streptococci
The nutritionally variant streptococci, previously S defectives and S adjacens and additional species, are now classified in the genus Abiotrophia and the genus Granulicatella. They have also been known as "nutritionally deficient streptococci" and "pyridoxal-dependent streptococci." They require pyridoxal or cysteine for growth on blood agar or grow as satellite colonies around colonies of staphylococci and other bacteria.

Peptostreptococcus (Many Species)
These streptococci grow only under anaerobic or microaerophilic conditions and variably produce hemolysins. They are part of the normal flora of the mouth, upper respiratory tract, bowel, and female genital tract. They often participate with many other bacterial species in mixed anaerobic infections. Such infections may occur in wounds, in the breast, in postpartum endometritis, following rupture of an abdominal viscus, the brain, or in chronic suppuration of the lung. The pus usually has a foul odor.

Streptococcus pneumoniae
Morphology & Identification
The pneumococci (S pneumoniae) are gram-positive diplococci, often lancet-shaped or arranged in chains, possessing a capsule of polysaccharide that permits typing with specific antisera. Pneumococci are readily lysed by surface-active agents, which probably remove or inactivate the inhibitors of cell wall autolysins. Pneumococci are normal inhabitants of the upper respiratory tract of 5–40% of humans and can cause pneumonia, sinusitis, otitis, bronchitis, bacteremia, meningitis, and other infectious processes.
Variation
Pneumococcal isolates that produce large amounts of capsules produce large mucoid colonies.
Antigenic Structure
The pneumococcal cell wall has peptidoglycan and teichoic acid, like other streptococci. The capsular polysaccharide is covalently bound to the peptidoglycan and to the cell wall polysaccharide.
Pathogenesis & Pathology
Pneumococcal infection causes an outpouring of fibrinous edema fluid into the alveoli, followed by red cells and leukocytes, which results in consolidation of portions of the lung. Many pneumococci are found throughout this exudate, and they may reach the bloodstream via the lymphatic drainage of the lungs. The alveolar walls remain normally intact during the infection.
Immunity
Immunity to infection with pneumococci is type-specific and depends both on antibodies to capsular polysaccharide and on intact phagocytic function. Vaccines can induce production of antibodies to capsular polysaccharides .
Treatment
Since pneumococci are sensitive to many antimicrobial drugs, Penicillin G is the drug of choice,
Epidemiology, Prevention, & Control
It is possible to immunize individuals with type-specific polysaccharides. Such vaccines can probably provide 90% protection against bacteremic pneumonia.

Enterococci
The enterococci have the group D group-specific substance and were previously classified as group D streptococci. Because the group D cell wall specific antigen is a teichoic acid, it is not an antigenically good marker; enterococci are usually identified by characteristics other than immunologic reaction with group-specific antisera. They are part of the normal enteric flora. They are usually nonhemolytic, but occasionally. Enterococci are PYR-positive. They grow in the presence of bile and hydrolyze esculin (bile esculin-positive). They grow in 6.5% NaCl. They grow well at between 10 °C and 45 °C whereas streptococci generally grow at a much narrower temperature range. They are more resistant to penicillin G than the streptococci. Many isolates are vancomycin-resistant.
There are at least 12 species of enterococci. Enterococcus faecalis is the most common and causes 85–90% of enterococcal infections, while Enterococcus faecium causes 5–10%. The enterococci are among the most frequent causes of nosocomial infections, particularly in intensive care units, and are selected by therapy with cephalosporins and other antibiotics to which they are resistant. Enterococci are transmitted from one patient to another primarily on the hands of hospital personnel, some of whom may carry the enterococci in their gastrointestinal tracts. Enterococci occasionally are transmitted on medical devices. In patients, the most common sites of infection are the urinary tract, wounds, biliary tract, and blood. Enterococci may cause meningitis and bacteremia in neonates. In adults, enterococci can cause endocarditis. However, in intra-abdominal, wound, urine, and other infections, enterococci usually are cultured along with other species of bacteria, and it is difficult to define the pathogenic role of the enterococci.




Other Catalase-Negative Gram-Positive Cocci


Table 2 Nonstreptococcal Catalase-Negative Gram-Positive Cocci and Coccobacilli.



Genus1
  Catalase Gram Stain Vancomycin Susceptibility Comment
Aerococcus  Negative to weakly positive Cocci in tetrads and clusters Susceptible Environmental organisms occasionally isolated from blood, urine, or sterile sites
Gemella  Negative Cocci in pairs, tetrads, clusters, and short chains Susceptible Decolorize easily and may look gram-negative; grow slowly (48 hours); part of normal human flora; occasionally isolated from blood and sterile sites
Leuconostoc  Negative Cocci in pairs and chains; coccobacilli, rods Resistant Environmental organisms; look like enterococci on blood agar; isolated from a wide variety of infections
Pediococcus  Negative Cocci in pairs, tetrads, and clusters Resistant Present in food products and human stools; occasionally isolated from blood and abscesses
Lactobacillus  Negative Coccobacilli, rods in pairs and chains Resistant (90%) Aerotolerant anaerobes generally classified as bacilli; normal vaginal flora; occasionally found in deep-seated infections


1Other genera where isolates from humans are rare or uncommon: alloiococcus, globicatella, helcococcus, lactococcus, tetragenococcus, vagococcus.