Viridans Streptococci, Nutritionally Variant Streptococci, and Groups C and G Streptococci

Viridans Streptococci

Viridans streptococci are gram-positive cocci arranged in pairs and chains and are facultative anaerobes, but some strains are microaerophilic. As a group viridans streptococci are fastidious organisms and grow best on blood agar but form small, raised colonies. They are nonmotile and nonspore formers. The viridans streptococci produce lactic acid, but not gas, from glucose. Colony size and characteristics vary by species.

Greenish discoloration surrounding viridans streptococcal colonies (“viridans” from the Latin virdis, meaning “green”) results in a greenish discoloration on blood agar that is due to partial red blood cell destruction. In general, the type of hemolysis distinguishes the viridans streptococci, but some strains may demonstrate γ-hemolysis or β-hemolysis. The intensity of α-hemolysis, that is, the intensity of greenish discoloration varies with the medium and viridans streptococcal species. Streptococcal colonies often glisten. After 24 hours of incubation, β-hemolytic streptococci (GAS, group C streptococci [GCS], and group G streptococci [GGS]) form large colonies (>0.5 mm in diameter); β-hemolytic small colonies suggest the S. anginosus group. These small S. anginosus (<0.5 mm in diameter) “pinpoint colonies” have honey or caramel odor. Group B β-hemolytic streptococcal colonies are larger and demonstrate less intense β-hemolysis (smaller zone) than other β-hemolytic streptococci. Colonies of viridans streptococci demonstrate α-hemolysis and have a “domed” appearance, in contrast to the flat colonies of other streptococci. The α-hemolytic colonies of S. pneumoniae typically have a depressed center. In general, viridans streptococci are not groupable by Lancefield antisera, but important exceptions include the Lancefield group F streptococci of the S. anginosus group.

Microbiologically, it is important to differentiate viridans streptococci from those in groups A, B, C, and G streptococci; group D enterococci; and nonenterococcal group D streptococci. S. pneumoniae is a lancet-shaped diplococcus and does not form long chains typical of the viridans streptococci. S. pneumoniae is α-hemolytic and demonstrates optochin inhibition.

Group D enterococci are bile soluble, grow in 6.5% NaCl broth, and are penicillin resistant. In contrast, S. bovis, (S. gallolyticus), a nonenterococcal group D streptococcus, is penicillin susceptible.

Streptococcus anginosus Group

The S. anginosus group differs from other viridans streptococci in its ability to cause invasive pyogenic infections, that is, particularly abscesses (brain, dental, lung, heart, liver, abdomen). Clinically, the S. anginosus group behaves more like Staphylococcus aureus than the other viridans streptococci, which are relatively avirulent. Viridans “small colony” streptococci (originally known as “minute hemolytic streptococci”), the S. anginosus group, may be rapidly differentiated from other viridans streptococci by three key tests: a positive Voges-Proskauer (VP) test, arginine hydrolysis, and inability to ferment sorbitol ( Table 202.3 ). If present, a deacetyl metabolite is responsible for the “caramel odor” and is a distinctive feature of S. anginosus colonies on solid media. Some isolates of the S. anginosus group may react with Lancefield A, C, F, or G group antisera, but if the isolate is Lancefield group F positive, then the organism is one of the S. anginosus group.

Nutritionally Variant Streptococci: Abiotrophia sp. and Granulicatella spp

The NVS have aspects in common with the viridans streptococci but have distinctive features. Like the viridans streptococci, they are commensals of the oropharynx and GI tract. Abiotrophia sp. and Granulicatella spp. differ from other viridans streptococci in their growth requirements. For NVS culture, media must contain thiol compounds ( l -cysteine or vitamin B ₆ or pyridoxamine). There is no growth on solid subculture media unless supplemented with pyridoxal (vitamin B ₆ ). Alternatively, using a S. aureus streak to provide thiol compounds, subculture produces satellite colonies (“satelliting”) surrounding the S. aureus streak. NVS colonies closest to the streak have uniform coccal morphology on Gram stain but show increasing pleomorphism the further from the S. aureus cross-streak. Aside from their vitamin B ₆ growth requirement, they are PYR positive, whereas the viridans streptococci are PYR negative. In blood cultures NVS appear as oval to bulbous bacilliform cocci in short chains resembling Streptobacillus moniliformis, and some have long filaments. Microbiologic clues to NVS on blood culture are gram-variable pleomorphic streptococci that fail to grow on unsupplemented solid media subculture.

Currently, NVS have been placed in two distinct genera consisting of four species: one Abiotrophia sp., that is, Abiotrophia defectiva, and three Granulicatella spp., that is, Granulicatella adiacens , Granulicatella elegans , and Granulicatella para-adiacens. Species are differentiated by hydrolysis of hippurate, arginine, galactosidase, glucuronidase, and carbohydrate fermentation.

Viridans Streptococci and the Streptococcus anginosus Group

The viridans streptococci are part of the normal microbiota of the mouth and GI tract. The concentration of viridans streptococci in the GI tract is greatest in the oropharynx. Colonization of the oral cavity varies by species, for instance, S. mitis and S. anginosus (buccal mucosa); S. oralis , S. mitis , and S. sanguinis (early dental plaque); Streptococcus gordonii (supragingival plaque); and S. anginosus (subgingival plaque). The adherence of the oral viridans streptococci is directly proportional to capsule production, which is related to fibronectin adherence to oral epithelial cells, that is, fibronectin selectively promotes adherence of oral streptococci, such as S. mutans . In hospitalized patients with impaired fibronectin function, decreased adherence of oral streptococci predisposes to colonization by other organisms, such as nosocomial gram-negative bacilli. The frequency of viridans streptococci causing SBE is directly related to their relative capsule production. During bacteremia originating in the oral cavity, viridans streptococci with polysaccharide capsules are able to adhere to damaged heart valves and initiate the development of valvular vegetations. After bacteremia adherence precedes colonization, and colonization precedes infection on damaged cardiac valves. Viridans streptococcal strains, most numerous in the oropharynx, with the most abundant capsule production are the most frequent species to cause SBE on damaged heart valves. Of interest, although the S. anginosus group is an oral commensal and associated with dental plaque as are the other oral viridans streptococci, the S. anginosus group is a rare cause of SBE. In addition, in the setting of decreased neutrophil numbers, such as neutropenia, in severely leukopenic oncology patients, viridans streptococci may cause primary bacteremia.

Nutritionally Variant Streptococci: Abiotrophia sp. and Granulicatella spp

The NVS were first described as a species closely related to S. mitior ( S. mitis ) but were found to be genetically unrelated. Abiotrophia sp . and Granulicatella spp., like viridans streptococci, are colonizers of the oropharynx, GI tract. As with other streptococci and staphylococci, some NVS strains may demonstrate “tolerance,” that is, minimal bactericidal concentration (MBC) ≥ 32 × minimal inhibitory concentration (MIC). If antibiotic “tolerance” is present, it has important therapeutic implications.

Viridans Streptococci

Except for S. mitis and S. pneumoniae , viridans streptococci lack the virulence factors of β-hemolytic streptococci, are relatively avirulent, and are not pathogenic for laboratory animals. To cause infection, viridans streptococci require impaired host defenses, for instance, severe leukopenia in oncology patients. Viridans streptococci are the most frequent pathogens in native valve SBE and late prosthetic valve endocarditis (PVE) pathogens. Viridans streptococci are not primary pathogens in meningitis, bone, skin, or urine infections. However, viridans streptococci are commonly cultured alone or with other colonizers and are often present in dental, hepatic, or GI abscesses. Before ascribing an infection to viridans streptococci and considering the causative organism a pathogen, the organism should be repeatedly isolated (in pure culture) from sterile body sites.

In viridans streptococcal SBE virulence is determined by lipoteichoic acid (fibronectin adhesin). After adherence to damaged cardiac valves, viridans streptococci promote platelet aggregation via tissue factor, which is the initial step in the formation of cardiac vegetations. Viridans streptococci are the most frequent pathogens causing native valve SBE in individuals with previous heart valve damage due to rheumatic heart disease, mitral valve prolapse, or degenerative valvular disease. Viridans streptococci are important pathogens in late prosthetic valve endocarditis (not early PVE). With normal heart valves, repeated bacteremias from the oral cavity are common but do not result in SBE. The ability of viridans streptococci to cause SBE is directly related to the amount of polysaccharide (dextran) capsule produced and is species dependent. Strains with more abundant capsules are the most frequent SBE pathogens; conversely, strains with little or no capsule, such as S. mitis, are infrequent causes of SBE.

S. mutans is particularly associated with dental caries. Adherence to dental enamel is the initial step in caries development and is mediated by glucan. S. mutans glucan-mediated colonization occurs and in the presence of fermented sucrose produces acid that damages the enamel.

S. anginosus group, unlike other viridans streptococci, produces a variety of hydrolytic enzymes, such as hyaluronidase and DNase. Some species are able to use sialic acid as a sole carbon source and may be a growth factor for these organisms. Some species, for instance, S. mitis, produce superantigens that stimulate nonspecific T-lymphocyte proliferation. Superantigens are potent inducers of cytokine release in systemic infections.

Streptococcus anginosus Group

Strains of S. anginosus group blunt chemotaxis and are relatively resistant to neutrophil killing. The mechanism of abscess formation by the S. anginosus remains unknown. The S. anginosus group rarely causes SBE, but when it does, infection is often complicated by myocardial abscess with heart block. Species of the S. anginosus group may be cultured from abscesses alone or with other viridans streptococci.

Nutritionally Variant Streptococci: Abiotrophia sp . and Granulicatella spp

Like viridans streptococci, NVS cause SBE on previously damaged heart valves. NVS causes SBE, which is often more severe than that caused by other viridans streptococci. Among NVS organisms, A. defectiva seems especially suited to cause endovascular infection because of its ability to adhere to extracellular fibronectin. Like other viridans streptococci, NVS may cause bacteremia in severely neutropenic oncology patients.