Abstract
We isolated atypical methicillin-resistant Staphylococcus aureus (MRSA) from two patients who were administered an antimicrobial drug for a prolonged period. These strains showed a typical colony on Colombian CNA 5% sheep blood (CNA:BD) medium for gram-positive cocci. However, these strains formed minute colonies on sheep blood agar M58 (Eiken). In addition, these strains were difficult to grow on MS-CFX agar (Nissui) as the selective medium for MRSA. Therefore, we observed the growth situation using a blood component agar and the selective MRSA medium under the conditions of aerobic culture, 5% carbon dioxide culture and anaerobic culture for five days. MRSA showed a typical colony on CNA and sheep blood agar M70 (Eiken) on the first day, but an atypical colony on the other agars. When we identified these strains using the Microbiology Systems Microscan WalkAway 96 SI (aerobic culture), they were unmeasurable owing to insufficient growth. When measurement panels cultured 5% carbon dioxide culture and anaerobiotic culture, they were miss-identified as Methicillin-sensitive Staphylococcus aureus and Staphylococcus capitis subsp. capitis. In the selective MRSA medium, no typical colony was observed. For detecting MRSA, we must be aware of the existence of atypical colonies, and it is necessary to culture MRSA in a nonselective medium in addition to the selective MRSA medium in the case of patients who were administered an antimicrobial drug for a prolonged period. When a growth difference in each agar is observed, we need to obtain the patient’s information and confirm it carefully by a method that does not use growth properties.
