with 49% on CHROMagar S. aureus and 66% on man- nitol salt agar. MRSA ID is adapted from S. aureus ID by the inclusion of cefoxitin, and this medium allowed the successful isolation of the strain re- ported here. 6 The fact that EMRSA-15 could not be detected in a tracheostomy swab using conventional culture media is a cause for concern from both a diagnostic and an infection control viewpoint. Resubmission of the thymidine-dependent variant in simulated samples as an internal quality assessment exercise confirmed that this strain could not be detected using conventional media. We hypothesize that auxotrophic variants of MRSA may potentially be an unrecognized reservoir of MRSA in the hospital setting, and suggest that further studies are warranted to examine the prevalence of such strains. Media such as blood agar, with or without selective enrichment, remain widely used for the detection of MRSA 7 but may not support the growth of such variants. 5 Microbi- ologists need to be aware of the possibility of such strains persisting in patients who have had long-term antimicrobial therapy, particularly with trimethoprim-sulphamethoxazole. In such pa- tients, laboratory methods may need to be varied to optimize recovery of these strains. 5,8 References 1. Acar JF, Goldstein FW, Lagrange P. Human infections caused by thiamine- or menadione-requiring Staphylococcus aureus. J Clin Microbiol 1978;8:142e147. 2. Seifert H, von Eiff C, Fatkenheuer G. Fatal case due to methicillin-resistant Staphylococcus aureus small colony vari- ants in an AIDS patient. Emerg Infect Dis 1999;5:450e453. 3. Gilligan PH, Gage PA, Welch DF, Muszynski MJ, Wait KR. Prevalence of thymidine-dependent Staphylococcus aureus in patients with cystic fibrosis. J Clin Microbiol 1987;25: 1258e1261. 4. Kahl B, Herrmann M, Everding AS, et al. Persistent infection with small colony variant strains of Staphylococcus aureus in patients with cystic fibrosis. J Infect Dis 1998;177:1023e 1029. 5. Kipp F, Kahl BC, Becker K, et al. Evaluation of two chromo- genic agar media for recovery and identification of Staphylo- coccus aureus small-colony variants. J Clin Microbiol 2005; 43:1956e1959. 6. Perry JD, Davies A, Butterworth LA, Hopley AL, Nicholson A, Gould FK. Development and evaluation of a chromogenic agar medium for methicillin-resistant Staphylococcus aureus. J Clin Microbiol 2004;42:4519e4523. 7. Wertheim HF, Vos MC, Boelens HA, et al. Low prevalence of methicillin-resistant Staphylococcus aureus (MRSA) at hospital admission in the Netherlands: the value of search and destroy and restrictive antibiotic use. J Hosp Infect 2004;56:321e325. 8. Kipp F, Becker K, Peters G, von Eiff C. Evaluation of different methods to detect methicillin resistance in small-colony variants of Staphylococcus aureus. J Clin Microbiol 2004; 42:1277e1279. V.J. Cleeve* J.D. Perry G. Cresswell K.E. Orr Department of Microbiology, Freeman Hospital, Newcastle upon Tyne, UK E-mail addresses: vickycleeve@blueyonder.co.uk, victoria.cleeve@nuth.nhs.uk Available online 4 April 2006 * Corresponding author. Address: Department of Microbiology, Freeman Hospital, Freeman Road, High Heaton, Newcastle upon Tyne NE7 7DN, UK. Tel.: þ44 191 2336161. ª 2005 The Hospital Infection Society. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jhin.2005.11.009 Meticillin-sensitive and -resistant Staphylococcus aureus: competition and co-carriage Madam, Kampf questioned the methodology used in our study on the possible protective effect of meticillin- sensitive Staphylococcus aureus (MSSA) against colonization with meticillin-resistant S. aureus (MRSA), and raised a number of interesting issues about the implication of our conclusion. 1,2 Kampf was concerned about the oxacillin concen- tration used to isolate MRSA from screening swabs. We classified an isolate as resistant if it grew on the primary plate of mannitol salt agar (MSA) containing 4 mg/L of oxacillin, which is still widely used for MRSA screening. The British Society of Antimicrobial Chemotherapy guidelines for the laboratory diagno- sis and testing of MRSA acknowledge that no single medium will recover all MRSA strains. 4 High sensitivity and specificity in detecting MRSA using MSA with an oxacillin concentration of 2 mg/L has been reported, 5 although studies with pure cul- ture from a collection of staphylococcal strains can- not be a substitute for comparative studies that assess clinical specimens. 4 Interestingly, data from Antibiotic Resistance: Prevention and Control (ARPAC) (available on the ARPAC website) indicate that, of the European centres that reported the use of oxacillin screening plates, 8.9% use an Letters to the Editor 229