Determinação da concentração inibitória mínima de Vancomicina e Oxacilina abordando diferentes métodos laboratoriais em isolados de Staphylococcus aureus de hemoculturas em hospital terciário

Abstract

Staphylococcus aureus is an important bacterium that causes hospital infections. A major problem in the treatment of diseases caused by this bacterium is the emergence of strains resistant to several antimicrobials, such as MRSA (methicillin-resistant S. aureus) and, more seriously, the emergence of VRSA (vancomycin-resistant S. aureus). It is important, therefore, to investigate the sensitivity profile of S. aureus strains in patients, especially those with bacteremia, and to obtain the minimum inhibitory concentration (MIC) of antibiotics. Currently, to obtain these values, automated methods are the most used in clinical laboratory practice due to their practicality and cost-benefit. However, these may have limited accuracy, for example for vancomycin. In this context, the present research aims to make a comparison between methods for detecting the MIC of these bacteria against vancomycin and oxacillin. The blood samples came from patients at a public tertiary hospital in Ceará from August 2022 to July 2023. A total of 62 blood cultures were positive for the bacteria during this period. Male individuals (64.52%) had a higher frequency of cases, but there was no difference related to sex regarding the distribution of strains, MRSA or methicillin-sensitive S. aureus (MSSA). The age group over 60 years old was the most present for both MRSA infections (16,13%) and MSSA strains (33,87%). Regarding the distribution of cases in the hospital environment, hospitalization units presented a percentage of 67.74% and intensive care units (ICU) 9.68%. Most of the patients involved had some related comorbidity, revealing yet another risk factor for infection. Of the positive blood cultures for S aureus, 37.10% were oxacillin-resistant strains. Overall, the MRSA phenotype showed the highest resistance rates, with the majority of MSSA strains (88.68%) being sensitive to all antibiotics. Benzylpenicillin was the antimicrobial with the highest resistance rate (100% for MRSA and 87.18% for MSSA). None of the isolates showed resistance to vancomycin. A total of 22 patients (35.48%) died as a result of as a clinical outcome. Regarding the comparison of methods, it was seen that for oxacillin, the MIC agreement between Vitek®2 and the Broth Microdilution (MDC) assay was 59.7% (r = 0.7106; p = <0.0001), while vancomycin was 79% (r = 0.6975, p = <0.0001). Exclusively for vancomycin, the manual Epsilometes test® (E-test®) method was added, obtaining an agreement of 38.7% between E-test® and Vitek®2 and 35.5% between E-test® and MDC. The E-test® MIC values (0.4637 ± 0.02289) were underestimated when compared to the MDC values (p > 0.05). Furthermore, the MDC and Vitek®2 methods showed positive and moderate correlations (r = 0.6975, p = <0.0001), while MDC and E-test® showed a positive and weak correlation (r = 0.2563; p = 0.0444). The vancomycin MIC determined by Vitek®2 showed a better correlation with the MDC MIC values (r = 0.6975, p = <0.0001) when compared to the E-test® MIC values (r = 0.2563; p = 0.0444). Therefore, there is a tendency for E-test® to underestimate vancomycin MIC values. However, even with better correlation and agreement, the results obtained by Vitek®2 must be evaluated with caution, as a tendency to overestimate the MIC of vancomycin by 1 (one) dilution was identified. The results showed that there may be variations in MIC values for vancomycin depending on the method used and that it is important to verify the importance of carrying out an additional test in order to protect the use of adequate therapy.