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Dolosigranulum pigrum, a gram-positive coccus arranged in pairs, tetrads, and clusters, was described by Aguirre et al. in 1993 (1). Growth of colonies on blood agar resembles that of viridans streptococci. Twelve of the 27 isolates of D. pigrum that the Centers for Disease Control and Prevention (CDC) Streptococcus Laboratory has received were isolated from blood cultures. Five cases of bacterial sepsis were reported, but none had subacute bacterial endocarditis noted in the accompanying clinical information. Six strains were isolated from eye cultures; one case of blepharitis was reported. Four strains were isolated from nasopharyngeal swabs, and one strain each was isolated from spinal cord autopsy, sputum, sinus, gastric, and urine specimens. The 27 isolates were tested against 15 antimicrobial agents by using the broth microdilution antimicrobial susceptibility method. Two cultures reported by Aguirre et al. (1) were isolated from the spinal cord of a patient with a fatal multiple sclerosis case and from a culture of an infected eye. We could not reproduce some of the test results reported by Aguirre et al. (1) in our laboratory. A discussion of these results and our final identification is given in reference 5.

To our knowledge, antimicrobial susceptibility testing for these bacteria has not yet been reported. The purpose of this communication is to broaden the knowledge of the clinical sources and diagnosis of disease caused by D. pigrum and to report the antimicrobial susceptibilities of these bacteria.

The strains tested were taken from the culture collection of the Streptococcus Laboratory at the CDC. The majority of cultures were submitted to the Streptococcus Laboratory for identification from various state health departments throughout the United States (see Table 1). Strains were identified according to previously described procedures (5). Based on 10 tests used to identify catalase-negative, gram-positive coccal genera, species of four genera have similar phenotypic profiles for genus identification by conventional tests: Alloiococcus otitidis, Ignavigranum ruoffiae, D. pigrum, and Facklamia species (3, 5). Alloiococcus differs from the Ignavigranum, Dolosigranulum, and Facklamia species by its aerobic nature (6). The majority of Dolosigranulum cultures are positive for esculin hydrolysis, while Iganvigranum and Facklamia species do not hydrolyze esculin. Specific identification has been published previously (5).

The MICs of 15 antimicrobial agents were determined by using microdilution in Mueller-Hinton broth supplemented with 3% lysed horse blood and the methods described by the NCCLS (7). The following antibiotics and concentration ranges were tested in a customized panel (PML Microbiologicals, Wilsonville, Oreg.): penicillin, 0.03 to 16.0 µg/ml; amoxicillin, 0.03 to 8.0 µg/ml; cefotaxime, 0.06 to 16.0 µg/ml; cefuroxime, 0.12 to 32 µg/ml; erythromycin, 0.06 to 16 µg/ml; trimethoprim-sulfamethoxazole, 0.12 and 2.38 to 8 and 152 µg/ml; clindamycin, 0.06 to 2.0 µg/ml; choramphenicol, 2.0 to 16.0 µg/ml; levofloxacin, 0.5 to 16 µg/ml; trovafloxacin, 0.25 to 8.0 µg/ml; meropenem, 0.06 to 2.0 µg/ml; vancomycin, 0.12 to 2.0 µg/ml; tetracycline, 1.0 to 8.0 µg/ml; quinupristin-dalfopristin, 1.0 and 4.0 µg/ml; and rifampin, 1.0 and 4.0 µg/ml (see Table 2). The panels were incubated under 5% carbon dioxide for 22 to 24 h. Several test strains grew poorly, if at all, when the panels were incubated aerobically. Panels were read visually with the aid of a mirror panel viewer.

Since these panels were designed to test MICs for Streptococcus pneumoniae, the panels were tested weekly for quality control with two control strains of S. pneumoniae: a multiply resistant strain (CDC317-79) and the ATCC type strain 49619 (as suggested by the NCCLS). During the 14-week period of testing of the D. pigrum isolates, the quality control values for penicillin were between 1 µg/ml (5×) and 2 µg/ml (9×) for the control strain CDC317-79 and 0.25 µg/ml (14×) for ATCC type strain 49619. The values for erythromycin for the same time period were between 2.0 µg/ml (2×) and 4.0 µg/ml (12×) for the control strain CDC317-79 and between 0.12 µg/ml (13×) and 0.6 µg/ml (1×) for ATCC type strain 49619. Although these values for the ATCC type strain 49619 are the lowest acceptable value for penicillin and the highest acceptable value for erythromycin, they are still within the values recommended by the NCCLS.

Based on distinctive results of biochemical tests, the identification of D. pigrum cultures is easier to establish compared with other members of this newly described unusual group of gram-positive cocci: Alloiococcus, Ignavigranum, and Facklamia species. The phenotypic characteristics of the four genera generated by vancomycin sensitivity, PYR (L-pyrrolidonyl--naphthylamide) hydrolysis, LAP (leucine aminopeptidase) hydrolysis, and growth in 6.5% NaCl broth, identify the four genera in this group (4). D. pigrum is the only species in this group that hydrolyzes esculin. Identification of the other members of this group can be problematic because of similar reactions generated with further biochemical tests (5).

Twelve of the 27 strains were isolated from blood cultures of ill patients, 5 of whom had a diagnoses of sepsis, indicating the importance of this bacterium and the seriousness of the types of infections it can cause (Table 1). Six of the 27 strains were isolated from eye infections. Interestingly, all six were resistant to erythromycin. Four isolates were found from our nasopharyngeal carriage surveys, which focused on detection of S. pneumoniae; this may indicate the natural habitat of this bacterium.

The ages of the patients from whom D. pigrum cultures were isolated ranged from 2 months to more than 80 years; most patients were either very young (under 2 years [five patients]) or very old (over 65 years [eight patients]). There were equal numbers of male and female patients.

None of the cultures were resistant to any of the beta-lactam antimicrobial agents tested. Resistance was noted mainly to erythromycin: 13 of the 27 strains required a MIC of either 1.0 or 2.0 µg/ml (Table 2). One strain, isolated from the nasopharynx of an asymptomatic child, was resistant to trimethoprim-sulfamethoxazole. These findings differ substantially from those for another newly described genus of bacteria, Facklamia (L. LaClaire and R. Facklam, submitted for publication). There is considerable antimicrobial resistance among Facklamia species (LaClaire and Facklam, submitted). Since it is likely that D. pigrum cultures were previously identified as Gemella species or Gemella-like bacteria, comparison of their antimicrobial susceptibilities to those of the Gemella species may be valid. In a review of the taxonomic and clinical importance of Gemella, only one previous report was referenced for the antimicrobial susceptibilities of Gemella species (8). The information available indicates that Gemella haemolysans is highly sensitive to the beta-lactam antimicrobics, and the only notable resistance found has been to trimethoprim-sulfamethoxazole (2). Although meager, these findings indicate that D. pigrum susceptibilities are similar to those of the Gemella species.

Of the 14 erythromycin-resistant strains, 6 were from cultures of the eyes of infected patients, 3 were from blood cultures of patients, (2 of whom had sepsis), 3 were from nasopharyngeal swabs of asymptomatic children, 1 was from the sputum of a patient with pneumonia, and 1 was from a spinal cord autopsy (Table 1).

We conclude that this gram-positive coccus is a potential pathogen for humans. It can cause a wide spectrum of disease; however, the incidence is unknown. The data support a propensity to cause eye infections, indicating some virulence properties. Notably, the majority of these strains demonstrate resistance to erythromycin.