MRSA cluster 1, comparison of antibiotic susceptibility and associated genetic elements
| Patient | Sample date | Antibiotic susceptibility | Gene presence/absence | ||||
| Penicillin | Tetracycline | blaZ | tetK | ||||
| DD | MIC | DD | MIC | ||||
| A | 25/7 | R | 2 | S | 0.094 | + | − |
| B | 27/7 | R | 2 | R | 24 | + | + |
| C | 27/7 | R | 4 | S | 0.094 | + | − |
| D (i) | 5/8 | R | 3 | R | 24 | + | + |
| D (ii) | 10/8 | R | 2 | R | 24 | + | + |
| E | 8/8 | R | 4 | R | 32 | + | + |
| F | 8/8 | S | 0.047 | S | 0.064 | − | − |
| G | 8/8 | R | 3 | S | 0.094 | + | − |
| H (i) | 28/9 | R | 4 | R | 24 | + | + |
| H (ii)* | 29/9 | R | 2 | R | 24 | + | + |
Screening swabs were obtained from patient D on two separate dates. All isolates were methicillin heteroresistant, appearing susceptible on routine testing, despite detection of mecA by PCR and sequencing. This explains why isolate F appeared phenotypically penicillin susceptible on DD and E testing. The mechanism of heteroresistance for these isolates has not yet been fully elucidated, although the penicillin-susceptible methicillin-resistant phenotype has been described.20
↵* The second isolate from patient H is from a positive blood culture.
DD, disc diffusion (R, resistant; S, susceptible); MIC, minimum inhibitory concentration (milligrams per litre).