Retrospective Study on the Prevalence and Antibiotic Resistance Pattern of Staphylococcus Aureus and Staphylococcus Epidermidis Among Patients Suspicious of Bacteremia During 2006-2011

1Department of Parasitology and Mycology, School of Medicine, Isfahan University of Medical Sciences, Isfahan, IR Iran 2Department of Microbiology and Virology, Mashhad University of Medical Sciences, Mashhad, IR Iran 3Department of Microbiology, School of Medicine, Hamadan University of Medical Sciences, Hamadan, IR Iran 4Department of Microbiology, School of Medicine, Isfahan University of Medical Sciences, Isfahan, IR Iran


Background
The staphylococci cause different diseases, varies from mild skin infections to serious bacteremia.They are a major cause of nosocomial and community-acquired infections present all over the globe (1).Staphylococcus aureus and Staphylococcus epidermidis are the two important opportunistic pathogens of the staphylococci.S. epidermidis is mostly common all over the cutaneous surfaces, whilst S. aureus is present rudimentarily on the mucosal membranes (2).
S. epidermidis can be distinguished from S. aureus by its inability to produce coagulase.It colonizes on the skin and mucosal membranes of the human body and represents the predominant part of the normal bacterial flora of skin.S. aureus is colonized in 30% to 50% of healthy adults, which is persistent in 10% to 20% (3).
Higher morbidity and mortality rates have been described in infections by organisms resistant to antibiotics comparing to antibiotic-susceptible ones (4).During last decades, infection with S. aureus has been increased, with mortality of 15% -60% in patients with staphylococcal bacteremia (5,6).The growing antibiotic resistance in staphylococcal and other bacterial infections is an emerging problem (6)(7)(8).

Objectives
To develop better and efficient health policies regarding treatment and control of the bacteremia, especially staphylococcal bacteremia, there is need for updated data about the prevalence of the infection and antibiotic resistance pattern in each region.This study aimed to investigate the prevalence of staphylococcal bacteremia caused by S. aureus and S. epidermidis and their antibiotic resistance pattern from patients referred to Ghaem Educational, Research and Treatment Center, Mashhad, Iran, during 6 years (2006 -2011).

Blood Culture and Bacterial Isolation
In this retrospective study, hospital medical records of 28000 patients of Ghaem Educational, Research and Treatment Center, Mashhad, Iran, who were suspicious of blood infections during 6 years (2005 -2011), were extracted.The sampling procedure was as follows; the patient's blood samples were cultured in biphasic brain heart infusion medium bottles.The cultured media were inoculated at 37˚C for 24 hours.Then they transferred to the subsequent media and biochemical tests were done for isolation of S. aureus and S. epidermidis as described before (9).

Antibiotic Susceptibility Test
The susceptibility of S. aureus and S. epidermidis isolates to 30 antibiotics (Pad Tan Teb, Iran) was carried out using disc diffusion method (10) and the test's procedure was done based on the manufacturer's construction.In brief the procedure is as follows: 1.5 × 108 CFU of bacterial emulsion, transferred to Muller-Hinton agar (Merck, Germany) and antibiogram discs were placed on the medium.The prepared media incubated for 18 hours at 35˚C.Interpretation of the results were performed using Clinical and Laboratory Standards Institute (CLSI) criteria (11).The results of antibiogram tests were reported as sensitive and resistance.

Discussion
S. epidermidis is a normal flora of the human skin (12).So contamination of needle during blood sampling must be considered in positive results and probably a percent of positive results may be biased in these kinds of studies.In the present study Staphylococcus spp.were isolated from 600 (2.14%) out of 28000 blood samples, from which 420 (70%) of the isolates were S. epidermidis.Only170 (28.3%) of the isolates identified as S. aureus.As the results shows S. epidermidis is much frequent in staphylococcal bacteremia than S. aureus.
Considering the antibiotic resistance pattern of the isolates, ampicillin, amoxicillin, cefixime, ceftazidime, penicillin, oxacillin, nalidixic acid and cephepime were the most antibiotics that the isolates were resistant against.Also vancomycin and chloramphenicol were the most effective antibiotics against S. aureus, and vancomycin was the most effective antibiotic against S. epidermidis.During 6 years increasing pattern in antibiotic resistance rate against some antibiotics can be seen.The antibiotic resistance pattern of the S. aureus and S. epidermidis during 2006 to 2011 are available in Tables 1 and 2, respectively.
As prevalence of infectious diseases have been decreasing during last decades (13), but the drug resistance became an important issue regarding the treatment of such diseases (7,8,14,15).Drug resistance in Staphylococcus spp. is increasing in its intensity and importance in health care settings (16).In the present study the high resistant rates against eight common antibiotics were observed among the isolates.
S. epidermidis and other coagulase negative staphylococci are reported to be serious cause of many nosocomial infections during the last two decades.The adherence and growing competency of staphylococci on plastic surfaces is a fundamental phase in the pathogenesis of infections associated with polymer.Various components that may be involved in slime production and adherence of Staphylococcus spp.have been described (17)(18)(19).
Methicillin resistant Staphylococcus aureus is one of the significant nosocomial pathogen with high mortality and morbidity in patients with suppressed and deficiencies of immune system (20).Also overuse of broad spectrum antibiotics for infections is a consequently increases the risk of acquiring the infections by resistant bacteria (20,21).High prevalence of antibiotic resistance bacterial infections can be associated to the overuse of antibiotics (22).Lari et al. (2000) reported S. aureus in 5.8% of blood cultures among burned patients of Tohid Burn Center in Tehran, Iran.In their findings S. aureus took second place after Pseudomonas aeroginosa as the most common cause of bacteremia among burned patients (23).Ekrami et al. (2007) studied bacterial infection of blood, urine and wound of 182 patients reffered to Taleghani Burn Hospital, Ahvaz, Iran.They reported S. aureus as the second common cause of nosocomial infection in burned patients.They noted 58 % of S. aureus isolates and 60% of coagulase negative Staphylococcus spp. as methicillin resistant, but these isolates were susceptible enough to teicoplanin and vancomycin (24).Rahimi et al. (2009) studied antibiotic resistance and isolation of methicillin resistance gene of S. aureus in Tehran hospitals.They identified 54.7% of their studied isolates as S. aureus.They reported 66, 65, 88, 88, 100, 41, 38, 41, 0, 40, 93, 20 and 64% of isolates resistant against kanamycin, cephotaxim, methicillin, oxacillin, ampicillin, erythromycin, clindamycin, sulphamethoxazoletrimethoprime, vancomycin, chloramphenicol, ciprofloxacin, gentamicin and tetracycline, respectively.All of their studied methicillin resistance S. aureus and 63% of intermediate resistant isolates were carried mecA gene (25).Our S. aureus isolates were highly resistante against ampicillin (85%), amoxicillin/ clavulanic acid (80%), cefixime (90.6%), ceftazidime (98.3%), penicillin (95%), oxacillin (97.7%), nalidixic acid (90.9%) and cephepime (100%).
Zarifian et al. ( 2014) studied the pattern of antibiotic resistance among S. aureus clinical isolates of Mashhad from 2009 to 2011.Their isolates were highly resistant to Ceftazidime (94%), Penicillin (91%), and Ampicillin (82%).Nearly all of their studied strains were susceptible against vancomycin (26).This study showed a resistance pattern close to our findings, which is not surprising, because the two studies carried out in the same city.
There is scarce data on blood bacteremia caused by Staphylococcus spp. in hospitals of Iran.Mohammadi et al. (2014) studied neonatal bacteremia isolates and their antibiotic resistance pattern in Sanandaj, Iran (27).They reported 7.6% positive for bacterial growth among 355 blood cultures from which 74% were Staphylococcus spp.In their study, the maximum resistance rate among Staphylococcus spp.observed against penicillin, and ampicillin, which is similar to our findings.Also 7.5% of their isolates were sensitive to vancomycin and ciprofloxacin, but our isolates were showed high sensitivity against these two antibiotics.
Prevalence of staphylococcal bacteremia caused by S. epidermidis is fairly high comparing to S. aureus among patients referred to Ghaem Educational, Research and Treatment Center, Mashhad, Iran.Also the resistance rate of Staphylococcus spp.isolated from blood against commonly used antibiotic is high, but there are some highly sensitive antibiotic against the infection.

Table 2 .
Antibiotic Resistance among S. epidermidis Strains Isolated from Patients with Bacteremia During 2006 -2011 a a All values are presented as No. (%).b Abbreviations: R, Resistance; S, Sensitive; ND, No Data.