Antibiotic susceptibility patterns of Staphylococcus aureus strains isolated at the Yaounde Central Hospital, Cameroon: a retro prospective study

Introduction Staphylococcus aureus is an important pathogen responsible for hospital and community acquired infection(s). Emerging resistance to methicillin in this organism has left physicians with few therapeutic alternatives to treat infections caused by it. This study was aimed at determining the antibiotic susceptibility patterns of Staphylococcus aureus strains isolated at the Yaounde Central Hospital, Cameroon. Methods from January 2014 to November 2016, a total of 250 non repeated strains were isolated from various clinical specimens. Isolates and antibiotic susceptibility profiles were identified through standard microbiological techniques. Results methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive Staphylococcus aureus (MSSA) accounted respectively for 80% (201/205) and 20% (49/205) of the total strains isolated. MRSA strains displayed high resistance to cefoxitin (100%), cotrimoxazole (89%), vancomycin (79.7%), lincomycin (70.3%), tobramycin (72.5%), doxycycline (68.0%), kanamycin (69.7%) and erythromycin (55.7%). In contrast, a high susceptibility was observed with rifampicin (82.6%). KTG (42.3%) and constitutive MLSB (17.4%) were the most frequent phenotypes recorded. Conclusion our results show that the carriage of acquired MRSA infections predominates in this population. Despite the noticeable multiresistance of MRSA strains to antibiotics, rifampicin remains the drugs of choice for the therapy of acquired MRSA infections in this setting. In order to slow down antimicrobial resistance, surveillance studies for antimicrobial susceptibility remains essential to identify resistance and inform policy on resistance.


Introduction
Staphylococcus aureus is one of the organisms most frequently encountered in hospital and community acquired infection, especially in elderly individuals [1]; rates of Staphylococcus aureus infection have increased during the past 2 decades [2]. Bacteremia due to S. aureus has been reported to be associated with mortality rates of 15%-60% [3]. Infections with antibiotic-resistant organisms are thought to result in higher morbidity and mortality rates than are similar infections with antibiotic-susceptible strains [4]. In the early 1940s the introduction of benzylpenicillin (penicillin G) temporarily solved the problem of staphylococcal infections, but the continued use of this agent caused the selection of resistant strains, which produced penicillinase (β-lactamase) [5]. Resistance to methicillin among S. aureus isolates is a growing problem and poses a significant threat for effective treatment of several difficult-to-treat infections in humans [6]. High mortality rates associated to MRSA  Isolation and identification of Staphylococcus aureus: the clinical specimens were inoculated onto plates of mannitol salt agar (MSA); they were incubated at 37°C for 24h. All colonies from primary culture were purified by subculturing onto MSA medium and incubating at 37°C for 24h to 48h [9]. Biochemical examination: biochemical tests were performed to confirm S. aureus using Catalase test, Coagulase test and DNase test.
Antibiogram pattern of S. aureus to some antimicrobial agents: the susceptibility of isolates to different anti-microbial agents was done by disk diffusion method using commercial disks [10] and interpreted according to EUCAST [11]. The results were recorded as susceptible(S), susceptible dose dependent (SDD) and resistant (R).
Pure colonies of S. aureus cultures were inoculated in peptone water and incubated at 37°C to get turbidity equal to 0.5 on the McFarland scale (108 CFU/ml). A sterile cotton swab was dipped into the inoculation, and the excess was removed by pressing the swab to the sides of the tube. The entire Mueller Hinton agar surface was swabbed. The inoculation was allowed to dry for 15 min. Antibiotic discs were applied on the medium. The plates were incubated at 37°C and examined after 18-24 h. The antimicrobial agents tested were resistance phenotypes were identified as described by Courvalin et al [12]. The quality control of discs used was performed using Staphylococcus aureus (ATCC 25923).
Data analysis: data were entered and analyzed using SPSS version 16.0 for windows (SPSS, Inc., Chicago, IL). Discrete variables were compared using the Chi-square test. Statistical significance difference was considered at value of p < 0.05.

Results
In total, 250 non repeated strains of S. aureus were included in the study. 19

Discussion
S. aureus, one of the most common nosocomial and communityacquired pathogens has now emerged as an ever-increasing problem due to its increasing resistance to several antibiotics. This study determined the susceptibility pattern of S. aureus strains isolated from different clinical specimen in a tertiary hospital to provide physicians with up to date information about the local data of antimicrobial resistance of this pathogen. Of the 250 strains of S. aureus studied, 19.6% (49/250) strains were methicillin susceptible and 80.4% (201/250) were methicillin resistant. This study showed a high rate of MRSA which seems to be similar to findings of 72% by Njoungang [1], 76% by Gonsu [13], 65.7% by Asiimwe [14], 30.7% by Akindolire [15] and 32.5% by Eshetie [16]. As compared to present findings, lower rate of MRSA have been reported by Eibach [17], Leibler [18] and Ravensbergen [19] who found 2%, 8.3% and 10% rate respectively. It has been noticed that the proportion of MRSA has increased worldwide since the past two decades and the rate varies  [23] and Swanston [24] where MRSA isolates were found susceptible to vancomycin. The high resistance of isolates to antibiotics could be due to the wide use of these drugs for the treatment of staphylococcal infections in our set up, as wide consumption of antibiotics results to the emergence of antibiotics resistant Staphylococcus species due to selective pressure.
In this study the rate of iMLSB among MSRA was found to be 4.5%.
Varying prevalence rates of iMLSB have been reported in different other studies; 2.1% from Brazil [25], 8% from Jamaica [26], 18.2% from Nepal [27], 25% from Cameroon [1], 28.6% from Iran [28], 20.7% [29] and 24.3% [30] from India. Higher iMLSB prevalence of 37.5% from India [31] and 91% from Japan [32] has also been reported. A comparatively low prevalence of inducible resistance in this study could be due to the geographical variations of circulatory clones. MRSA isolates showed high rates of resistance to aminoglycoside agents. The study of resistance phenotypes of our MRSA to these drugs showed three types. 9 strains (4.5%) had a phenotype K, due to the production of the enzyme-Aminoglycoside phosphotransferase APH (3')-III. 30(14.9%) were resistant to kanamycin and tobramycin, KT is the phenotype expressed by the production of the enzyme-Aminoglycosides nucleotidyltransferases ANT (4') (4"), while 85 strains (42.3%) expressed the KTG phenotype and were resistant to the three antibiotics (kanamycin, tobramycin, gentamicin) due to the bifunctional enzyme APH (2") -Aminoglycosides acetyltransferases AAC (6') [33]. Our results are supported by those of Ida et al [34] and Shaw et al [35] who showed that most MRSA isolates seemed likely to produce AAC(6')/APH(2"), with or without ANT(4')-I, among the five kinds of aminoglycoside-modifying enzymes. In general, a beta-lactam or vancomycin often is combined with an aminoglycoside due to their synergistic effect and increased rates of killing in serious staphylococcal infections [36]. However, considering the high rates of resistance to the aminoglycosides, the addition of an aminoglycoside for the treatment of MRSA infections may be unpredictable in our set up.

Conclusion
The

Competing interests
The authors declare no competing interest.

Acknowledgments
Special thanks are due to the Director of the Yaounde Central Hospital who granted the research authorization for this study. Table 1: distribution of S. aureus by gender