Comparison of minimum inhibitory concentrations to methicillin in heterogeneous and homogeneous methicillin resistant Staphylococcus aureus.

OBJECTIVE: To determine the minimum inhibitory concentration (MIC) to methicillin and compare it with the type of methicillin resistance in methicillin resistant Staphylococcus aureus (MRSA) isolated from clinical samples, and to evaluate the usefulness of beta-lactamase resistant penicillins such as cloxacillin for treatment of minor MRSA infections. DESIGN: Sixty-five strains of MRSA isolated from clinical samples were studied. Homogeneous and heterogeneous resistance to methicillin of these strains was determined by an efficiency of plating technique. The MICs to methicillin of all homogeneously resistant strains and an equal number of heterogeneously resistant strains were determined by the 'E' test. SETTING: General Hospital, Colombo; De Soysa Maternity Hospital for Women; Lady Ridgeway Hospital for Children. RESULTS: 171 strains of S aureus were isolated from specimens processed in the laboratory of the Department of Microbiology, Faculty of Medicine Colombo, during a 14-month period. Seventy (40.9%) were methicillin resistant. Fifty-three MRSA strains (81.5%) were heterogeneously resistant to methicillin. Only 12 of 65 strains were homogeneously resistant. Nine of 12 of the homogeneously resistant strains were from surgical wards. MICs for methicillin in the two groups were compared. Six of 12 (50%) homogeneously resistant strains had very high levels of resistance to methicillin (MIC > 200 micrograms/ml). In contrast the MICs of all the heterogeneously resistant strains ranged from 16 micrograms/ml to 96 micrograms/ml. CONCLUSIONS: The majority of clinical isolates of MRSA from hospitals in Colombo, Sri Lanka were heterogeneously resistant to methicillin with lower MICs for methicillin than the homogeneously resistant strains. Thus, beta-lactamase resistance penicillins such as cloxacillin may have a place in the treatment of minor MRSA infections. Surveillance studies to determine if there is an increase of homogeneously methicillin resistant MRSA are required.

Emergence of penicillin resistant pneumococci in Sri Lanka.

Two strains of penicillin resistant (PR) pneumococci were isolated recently from cerebrospinal fluid of two children with meningitis. Penicillin resistance was detected by using a lug disc of oxacillin in the Stokes disc diffusion method. The strains were tested quantitatively for sensitivity to penicillin by an agar dilution technique. The minimum inhibitory concentration (MIC) to penicillin of these strains were 2 micrograms/ml and 0.12 microgram/ml, indicating that respectively they were of frank and intermediate resistance to penicillin. However these strains were sensitive to other antibiotics tested, namely, erythromycin, tetracycline, chloramphenicol, amoxycillin, cephalexin and cefotaxime. Penicillin resistant pneumococci are here in Sri Lanka. Though one strain was of intermediate resistance, therapeutic failure during treatment with penicillin was evident in this case.

Susceptibility to rubella virus among Sri Lankan women.

Immunity to the rubella virus was tested in 534 women in the childbearing age by the haemolysis-in-gel test, and 228 (43%) were found to be susceptible. This level of susceptibility is higher than that found in most temperate countries and mainland populations, and similar to descriptions in a few island and rural populations in the tropics. Although rubella epidemics do not occur in Sri Lanka, cases of congenital rubella and rubella complicating pregnancy are diagnosed. But the risk of congenital deformity in the foetus is not sufficient cause for a legal abortion in Sri Lanka. Under the circumstances it appears appropriate to use the measles--mumps--rubella vaccine rather than the measles vaccine for vaccination of infants or to introduce rubella vaccination of teen-age girls or female school leavers. Furthermore wherever possible, primipara could be screened by inexpensive methods such as the haemolysis-in-gel test and offered rubella vaccination after partus.