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Journal of Clinical Microbiology, February 2007, p. 685-686, Vol. 45, No. 2
0095-1137/07/$08.00+0     doi:10.1128/JCM.02228-06

LETTER TO THE EDITOR

Identification of Staphylococcus Species with the VITEK 2 System: the Case of Staphylococcus hominis


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LETTER
 
We read with interest the paper by Layer et al. regarding the identification of staphylococci with automated systems including VITEK-2 (bioMérieux, Marcy l'Etoile, France) (6). Of 86 strains analyzed in their study, 4 were Staphylococcus hominis and all were correctly identified by the colorimetric ID-GP VITEK-2 cards. Identical results have been reported in recent studies evaluating 13 and 14 S. hominis isolates (3, 7).

Two S. hominis subspecies exist—subspecies hominis and subspecies novobiosepticus—which may be differentiated according to novobiocin susceptibility (5). S. hominis subsp. hominis is more common in clinical samples, but S. hominis subsp. novobiosepticus has recently been shown to cause bacteremia among neonates (1). VITEK-2 actually identifies S. hominis with "low discrimination," and performance of manual novobiocin susceptibility testing is advised by the manufacturer in order to differentiate S. hominis subsp. hominis and S. hominis subsp. novobiosepticus. Thus, it would be interesting to know what subspecies were used by Layer et al. and whether the system performed similarly with both subspecies.

Interestingly, novobiocin resistance testing is one of 43 reactions carried out by the ID-GP card. While a negative result may represent susceptibility but also early termination of the reaction, it is tempting to regard a positive reaction as suggestive of S. hominis subsp. novobiosepticus. We studied nonduplicate consecutive clinical isolates of S. hominis and S. epidermidis (15 each) identified by VITEK-2. S. epidermidis was chosen as the control since it is almost always novobiocin susceptible (4). Isolates were processed by using VITEK-2 according to the manufacturer's instructions. Isolates were inoculated onto cation-adjusted Mueller-Hinton agar (Hy-Labs, Rehovot, Israel) according to Clinical Laboratory Standards Institute guidelines (2) and tested with a 5-µg novobiocin disk (BBL, BD Diagnostic Systems, Cockeysville, MD). A zone-of-inhibition diameter of ≤15 mm was considered to indicate resistance (5). Quality control was monitored using S. aureus ATCC 29213.

According to VITEK-2, of 15 S. hominis isolates, 5 (33.3%) were resistant to novobiocin, 8 were susceptible, and 2 yielded an indeterminate result. Of 15 S. epidermidis isolates, 3 (20%) were novobiocin resistant and 12 (80%) were susceptible. However, by disk diffusion, all isolates of both species were novobiocin susceptible. The mean zone-of-inhibition diameter was 34.3 ± 4 mm overall (range, 26 to 43 mm). That for S. hominis was 31.8 ± 3.1 mm (range, 26 to 36 mm), and that for S. epidermidis was 36.8 ± 3.2 mm (range, 33 to 43 mm; P, <0.001 by the Kruskal-Wallis test). The mean zone diameter for the eight isolates that were novobiocin resistant by VITEK-2 was 34.5 ± 3.4 mm, compared to 34.9 ± 3.8 mm for the 20 susceptible strains (P = 0.98). Results were similar when S. hominis and S. epidermidis were analyzed separately.

Novobiocin susceptibility results from the VITEK-2 ID-GP cards do not correlate well with disk diffusion results, and manual testing is indeed warranted for differentiating S. hominis subsp. hominis and S. hominis subsp. novobiosepticus. Resolving the low-discrimination result obtained with S. hominis is of even greater value given that S. hominis subsp. novobiosepticus is not included in the VITEK-2 advanced expert system knowledge base and, therefore, therapeutic corrections are not suggested for S. hominis subsp. novobiosepticus. Given all of the above, we believe that in future studies evaluating the performance of automated systems with coagulase-negative staphylococci, the issue of S. hominis subsp. hominis and S. hominis subsp. novobiosepticus differentiation should receive more emphasis.


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REFERENCES
 
    1
  1. Chaves, F., M. Garcia-Alvarez, F. Sanz, C. Alba, and J. R. Otero. 2005. Nosocomial spread of a Staphylococcus hominis subsp. novobiosepticus strain causing sepsis in a neonatal intensive care unit. J. Clin. Microbiol. 43:4877-4879.[Abstract/Free Full Text]
    2. 2
  2. Clinical Laboratory Standards Institute. 2006. Performance standards for antimicrobial susceptibility testing; 16th informational supplement. M100-S16. Clinical Laboratory Standards Institute, Wayne, PA.
    3. 3
  3. Funke, G., and P. Funke-Kissling. 2005. Performance of the new VITEK 2 GP card for identification of medically relevant gram-positive cocci in a routine clinical laboratory. J. Clin. Microbiol. 43:84-88.[Abstract/Free Full Text]
    4. 4
  4. Janda, W. M., K. Ristow, and D. Novak. 1994. Evaluation of RapiDEC Staph for identification of Staphylococcus aureus, Staphylococcus epidermidis, and Staphylococcus saprophyticus. J. Clin. Microbiol. 32:2056-2059.[Abstract/Free Full Text]
    5. 5
  5. Kloos, W. E., C. G. George, J. S. Olgiate, L. Van Pelt, M. L. McKinnon, B. L. Zimmer, E. Muller, M. P. Weinstein, and S. Mirrett. 1998. Staphylococcus hominis subsp. novobiosepticus subsp. nov., a novel trehalose- and N-acetyl-D-glucosamine-negative, novobiocin- and multiple-antibiotic-resistant subspecies isolated from human blood cultures. Int. J. Syst. Bacteriol. 48:799-812.[Abstract/Free Full Text]
    6. 6
  6. Layer, F., B. Ghebremedhin, K. A. Moder, W. Konig, and B. Konig. 2006. Comparative study using various methods for identification of Staphylococcus species in clinical specimens. J. Clin. Microbiol. 44:2824-2830.[Abstract/Free Full Text]
    7. 7
  7. Wallet, F., C. Loiez, E. Renaux, N. Lemaitre, and R. J. Courcol. 2005. Performances of VITEK 2 colorimetric cards for identification of gram-positive and gram-negative bacteria. J. Clin. Microbiol. 43:4402-4406.[Abstract/Free Full Text]
Jacob Gilad*
David Schwartz
Microbiology Laboratory
Tel-Aviv Sourasky Medical Center
6 Weizman St.
Tel-Aviv 62439, Israel

* Phone: 972-57-8125093, Fax: 972-3-6973663, E-mail: giladko{at}014.net.il


Authors' Reply


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LETTER 
 
We read the letter by Gilad and Schwartz regarding our publication (4), and we are glad to comment on their observations about the species Staphylococcus hominis subsp. hominis and S. hominis subsp. novobiosepticus. In our strain collection for the publication (4), we analyzed four S. hominis strains. Identification by VITEK-2 revealed S. hominis without identification to the subspecies level. But the confidence level for the species identification was 99%. The two S. hominis subspecies—subspecies hominis and subspecies novobiosepticus—can be differentiated according to novobiocin susceptibility. Therefore, we identified to the subspecies level the respective S. hominis strains and six additional clinical isolates of S. hominis (n = 10) by using novobiocin susceptibility testing and one reference strain of S. hominis subsp. hominis (CCM2732). Additionally, as a control for novobiocin susceptibility, we tested 12 clinical isolates of S. epidermidis as novobiocin sensitive (2) and one reference strain of S. saprophyticus subsp. saprophyticus as novobiocin resistant (2). Isolates were inoculated onto Mueller-Hinton agar (Oxoid Ltd., Basingstoke, Hampshire, England) according to Clinical Laboratory Standards Institute guidelines (1) and tested with a 5-µg novobiocin disk (Oxoid, Basingstoke, Hampshire, England). A zone-of-inhibition diameter of <15 mm was considered to indicate resistance (3).

We did not observe novobiocin resistance in all S. hominis strains tested, including the reference control strain of S. hominis subsp. hominis. The clinical control strains of S. epidermidis were all novobiocin sensitive, and the reference control strain of S. saprophyticus subsp. saprophyticus was novobiocin resistant by the disk diffusion test (Table 1).


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  		TABLE 1. Novobiocin susceptibility test and identification results from VITEK-2

Novobiocin resistance testing is one of 43 reactions carried out by the GP card of the VITEK-2 system. All isolates were processed by using VITEK-2 according to the manufacturer's instructions. In our hands, novobiocin susceptibility results from the VITEK-2 ID-GP card correlated with disk diffusion results by manual testing.

So far, in our clinical setting we have not observed any occurrence of S. hominis subsp. novobiosepticus, but in future studies we will look out for the issue of the subspecies differentiation for S. hominis.


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REFERENCES 
 
    1
  1. Clinical Laboratory Standards Institute. 2006. Performance standards for antimicrobial susceptibility testing; 16th informational supplement. M100-S16. Clinical Laboratory Standards Institute, Wayne, PA.
    2. 2
  2. Janda, W. M., K. Ristow, and D. Novak. 1994. Evaluation of RapiDEC Staph for identification of Staphylococcus aureus, Staphylococcus epidermidis, and Staphylococcus saprophyticus. J. Clin. Microbiol. 32:2056-2059.[Abstract/Free Full Text]
    3. 3
  3. Kloos, W. E., C. G. George, J. S. Olgiate, L. Van Pelt, M. L. McKinnon, B. L. Zimmer, E. Muller, M. P. Weinstein, and S. Mirrett. 1998. Staphylococcus hominis subsp. novobiosepticus subsp. nov., a novel trehalose- and N-acetyl-D-glucosamine-negative, novobiocin- and multiple-antibiotic-resistant subspecies isolated from human blood cultures. Int. J. Syst. Bacteriol. 48:799-812.[Abstract/Free Full Text]
    4. 4
  4. Layer, F., B. Ghebremedhin, K. A. Moder, W. König, and B. König. 2006. Comparative study using various methods for identification of Staphylococcus species in clinical specimens. J. Clin. Microbiol. 44:2824-2830.[Abstract/Free Full Text]
F. Layer
B. Ghebremedhin
K. A. Moder
W. König
B. König
Otto-von Guericke University
Institute of Medical Microbiology
Leipziger Str. 44
D-39120 Magdeburg, Germany

Journal of Clinical Microbiology, February 2007, p. 685-686, Vol. 45, No. 2
0095-1137/07/$08.00+0     doi:10.1128/JCM.02228-06





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