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Other Gram-Positive, Catalase-Negative Cocci: Leuconostoc and Pediococcus Species and Other Genera
Characteristics Of Pathogens
Among the gram-positive catalase-negative cocci, streptococci and enterococci cause most human infections. The remaining organisms belong to 5 genera that can be identified according to the criteria proposed by Facklam and colleagues ( Table 124.1 ), and they form colonies that are α hemolytic or nonhemolytic on sheep blood agar and are easily confused with Streptococcus pneumoniae or viridans streptococci. These organisms appear to lack typical virulence determinants and consequently are uncommon pathogens.
Table 124.1
Biochemical Differentiation of Catalase-Negative Gram-Positive Cocci
Adapted from Facklam R, Hollis D, Collins D. Identification of gram-positive coccal and coccobacillary vancomycin-resistant bacteria. J Clin Microbiol. 1989;27:724–730.
| Test | Streptococcus | Enterococcus | Pediococcus | Leuconostoc | Aerococcus | Gemella | Lactococcus |
|---|---|---|---|---|---|---|---|
| Arrangement of cells | Chains, pairs | Pairs, chains | Tetrads, some pairs | Pairs, chains | Tetrads, pairs | Pairs, short chains | Pairs, short chains |
| Gas from glucose | − | − | − | + | − | − | − |
| Vancomycin susceptibility | S | S a | R | R | S | S | S |
| Streptococcal group antigen | V | + | + | + a | − | − | − |
| Bile esculin | V | + | + | + a | V | − | V |
| PYRase b | − c | + | − | − | + | + | V |
| Leucine aminopeptidase | + | + | + | − | − | + | + |
| Growth in | |||||||
| 65% NaCl | − | + | V | V | + | − | V |
| 45°C | V | + | + a | − | − | − | − a |
| 10°C | − | + a | − | + | − | − | + |
NaCl, sodium chloride; PYRase, pyrrolidonyl peptidase; R, resistant to vancomycin (absence of a zone of inhibition); S, susceptible to vancomycin; v, variable reaction; −, absent; +, present.
a Occasional exceptions exist.
b Hydrolysis of L-pyrrolidonyl-β-naphthlyamide.
c Except Streptococcus pyogenes, which is pyrrolidonyl peptidase positive.
Of these genera, Leuconostoc and Pediococcus account for most human infections and are notable for their intrinsic high-level resistance to vancomycin. The genus Leuconostoc consists of 6 species, of which L. mesenteroides and L. paramesenteroides are most commonly associated with pediatric infections. The genus Pediococcus consists of 8 species, but only P. acidilactici and P. pentosaceus are recognized as potential pathogens.
Epidemiology
Leuconostocs and pediococci are members of the lactic acid bacteria and are found on certain foods, including vegetables, dairy products, and processed meats. Occasionally they can be isolated from culture of vaginal and gastrointestinal tract specimens. Leuconostocs can colonize other mucosal surfaces as well, including gastrostomy and tracheostomy sites. Pediococcus and Leuconostocs are components of several probiotic preparations.
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