An increase in the emergence of CRE, including Enterobacter spp. leaving limited treatment options, therefore are related to high morbidity and mortality [3, 4, 8]. In the current study, we determined antimicrobial resistance pattern and the presence of integrase and oxacillinase genes among 90 Enterobacter clinical isolates obtained from a tertiary care hospital from south of Iran. According literature, E. cloacae and E. aerogenes are the most prevalent clinical isolates of Enterobacter . In consistent with literature, in our survey E. aerogenes and E. cloacae were also the most common species with frequencies 45.6 and 30%, respectively. On the contrary, in the study of Khashei et al., E. gergoviae (54.2%) was found the most frequently isolated species . Enterobacter isolates have been the leading cause of nosocomial infections all over the world [1, 2]. In our work, most of isolates (37.8%) were recovered from respiratory tract samples. In agreement with our survey, in two study from China and Germany 91 and 37.8% of isolates were obtained from RTIs, respectively [10, 26]. In contrast, in several investigations blood and abdominal samples were the most frequent sites of Enterobacter isolation [8, 27, 28]. In this study, 64.4% of isolates were recovered from ICU ward. Likewise, some studies nationwide shared similar findings [26, 29]. This issue indicates the importance of long-term hospitalization for acquisition of these infections.
In susceptibility testing, all isolates were resistant to all of the tested antimicrobials, with 93% of strains showing MDR phenotype, making them a major therapeutic treat in our area. Based on CLSI, resistance to one or more carbapenems is considered as carbapenem resistance . Hence, 27.8% our isolates were non-susceptible to imipenem (carbapenem resistant). The result is coinciding with two previously reported works from Iran and China with prevalence’s of 29.2 and 25.7%, respectively [16, 30]. Moreover, among carbapenem-resistant isolates, 96% were phenotypically carbapenemase producers by mCIM test. The mCIM is a reliable and simple method and has a sensitivity of 98–100% in comparison with the modified Hodge test and the carbapenemase Nordmann-Poirel (carba NP) test with sensitivities of 93–98% and 73–100%, respectively. Furthermore, due to being expensive of PCR, mCIM could be a suitable alternative method for detecting of carbapenemase producers [12, 31–33].
In addition to rapid phenotypic tests, genotypic identification of carbapenemases among Gram-negative rods is an important step for infection control and prevention. The major mechanism of resistance to carbapenems among Enterobacter isolates is carbapenemase production, mainly OXA-type carbapenemases, including blaoxa−1, blaoxa−2, blaoxa−10 and blaoxa−48 -like genes and metallobetalactamases with a less prevalence . In the current work, the presence of blaoxa−1, blaoxa−2, blaoxa−10 and blaoxa−48 -like genes were diagnosed with frequencies 6.7, 5.6, 20, and 20%, respectively. The presence of oxacillinase genes in Enterobacter spp. are less than other Enterobacteriaceae members and their prevalence varies across the world. The blaoxa−48 first time identified among Klebsiella pneumoniae isolates in 2004 from Turkey. Afterward, the gene reported in some other Enterobacterales, including E. cloacae in Middle East, Africa and Europe [34, 35]. But this gene along with other oxacillinase genes such as blaoxa−10-like has not been reported among Enterobacter isolates in our region, yet. To the best of our knowledge, this will be the first reporting on Enterobacter spp. harboring blaoxa−10 and blaoxa−48-like genes from Iran.
In three studies conducted in Russia, Turkey and Germany, the prevalence of blaoxa−48 among Enterobacter isolates was reported as 20, 34.8 and 10.7%, respectively [36–38]. In addition, our results revealed that 6.7% and 5.6% of isolates were positive for blaoxa−1 and blaoxa−2-like genes, correspondingly. The result was less slightly than those observed by Ramezanzadeh and colleague with prevalence’s of 7.7% (blaoxa−1) and 11.8% (blaoxa−2) . In this survey, we found only 16.6% of mCIM positive isolates were carried blaoxa−48 -like gene, indicating other mechanisms such as metallobetalactamases or OmpK gene that regulates expression of mutated efflux pump are involved in carbapenem resistance. These discrepancies on phenotypic and genotypic results are also cited by some authors [13, 33, 40–42].
On the other hand, we identified the presence of class 1, 2 and 3 integrons among our isolates. Carbapenemase genes are harbored on mobile genetic elements and can be disseminated to other bacteria, therefore contributing to the origin of antimicrobial resistance among Enterobacteriaceae members . It is reported that class 1 integron is found in 40–70% of Gram-negative pathogens and has had a major role in widespread of antibiotic resistance [14, 15]. In consistent with literature, intI1 was found in the highest rate (55.6%), followed by intI2 (13.3%) and 8.9% of isolates were positive for both classes of integron genes, simultaneously. However, a significant difference was determined between isolates harboring intI2 gene and higher rate of drug resistance with only two antibiotics (Table 3). In accordance to our results, Mortazavi et al. showed that 58.3% of E. cloacae isolates, harbored class I integron; however, none of them had class II integron . In the present work, the intI3 gene was not detected in any of the isolates. It is mentioned that distribution of class 3 integron has been limited within a few Gram-negative bacteria, with the exception of Enterobacter spp. ranged from 0–10%, which is according to our findings .
This investigation had several limitations. First, the sample size was relatively small. Second, due to lack of temocillin disk (30 µg) we could not phenotypically diagnose OXA-48-producing isolates. Third, we could not evaluate the presence of more OXA-type carbapenemase genes to better assess of oxacillinase resistance among our isolates.