Changing patterns among the subgroups of strains of Staphylococcus aureus of phage group II in Danish hospitals from 1961-91

Penicillin-susceptible Staphylococcus aureus: susceptibility testing, resistance rates and outcome of infection

BACKGROUND

Staphylococcus aureus (SA) is an important human pathogen that causes both superficial and invasive infections. Penicillin is now rarely used in the treatment of SA infections due to widespread resistance and a concern about the accuracy of existing methods for penicillin susceptibility testing. The aims of the present study were to determine the frequency of penicillin-susceptible SA isolates from blood and wound cultures in Lund, Sweden, and to evaluate methods for penicillin testing in SA. We also wanted to investigate if penicillin-susceptible isolates are associated with higher mortality.

METHODS

Hundred blood culture isolates collected 2008/2009, 140 blood culture isolates from 2014/2015, and 141 superficial wound culture strains from 2015 were examined. Penicillin susceptibility was tested with disk diffusion according to EUCAST guidelines, and results were confirmed with a cloverleaf assay and PCR amplification of the BlaZ gene. Patient data for all bacteraemia cases were extracted from medical records.

RESULTS

The disk diffusion method with assessment of both zone size and zone edge appearance had high accuracy in our study. About 57% of bacteraemia isolates from 2008/2009 were sensitive to penicillin compared to 29% in 2014/2015 (p < .0001). In superficial wound cultures, 21% were penicillin susceptible. There was no difference in co-morbidity or mortality rates between patients with penicillin resistant and penicillin sensitive SA bacteraemia.

CONCLUSION

Disk-diffusion is a simple and reliable method to detect penicillin resistance in SA, and susceptibility rates are significant. Penicillin has many theoretical advantages and should be considered in the treatment of SA bacteraemia when susceptible.

Carriage of Staphylococcus aureus among 104 healthy persons during a 19-month period

The present study was undertaken to investigate the frequency of the nasal carrier rate of Staphylococcus aureus. The investigation was performed on 104 healthy persons. The total number of swabs performed was 1498 and this resulted in isolation of 522 S. aureus strains. All strains have been identified, tested for antibiotic susceptibility, and phage-typed. The carrier-index (number of positive swabs/number of total swabs for each individual person) was compared with different sampling and culturing methods, phage type, age, and resistance to antibiotics. There was statistical difference in carrier rate according to sex (P < 0.05). Among the 104 persons 15 (14.4%) were persistent carriers, 17 (16.3%) intermittent carriers, 55 (52.9%) occasional carriers and 17 (16.3%) non-carriers. Among intermittent and occasional carriers the phage-type distribution was different from the S. aureus strains isolated from Danish hospitalized patients in 1992, while the persistent carriers had similar phage-type distribution.

beta-Lactamase production and genetic location in Staphylococcus aureus: introduction of a beta-lactamase plasmid in strains of phage group II

Staphylococcus aureus strains of phage group II have increased in frequency in hospital-acquired infections during recent years. A total of 184 penicillin resistant group II strains from bacteraemia cases in the years 1961-1990 were analysed for the amount of beta-lactamase produced and the location of the beta-lactamase gene. Until 1977 all strains investigated had a chromosomally located beta-lactamase gene, but since then a 21 kb beta-lactamase plasmid has increased in occurrence among group II strains, especially among those strains typable only at high phage concentrations [100 x Routine Test Dilution (RTD) and 1000 x RTD]. In 1990, 84% of the group II strains contained this plasmid. Plasmid-containing strains produced more beta-lactamase than strains without the plasmid. S. aureus strains of the 94,96 complex, which since 1984 have decreased in frequency from 18 to 9% in 1993, have remained high producers of beta-lactamase.