Enterococci have long been considered as one of the most common causes of nosocomial infections. The rise of drug-resistant strains presents a serious problem to control in enterococcal infections. Several resistant Enterococcus species have been reported, including E. faecalis, E. faecium E. avium, E. durans, E. gallinarum, E. casseliflavus, E. raffinosus, E. mundtii, E. malodoratus, and E. hirae [12]. As previously found in Egypt [5, 13], E. faecalis and E. faecium are the predominant strains revealed in all clinical specimens in the present study.
Aminoglycosides are considered efficient in treating serious infections caused by both Gram-positive and Gram-negative organisms. However, the acquisition of extrinsic resistance to high-level aminoglycoside antibiotics in enterococci renders these strains a serious challenge in clinical settings [14].
Distribution of HLAR in enterococcal isolates varied in different reports in the world. The current study results (41.7%) were comparable to those obtained by El-Ghazawy et al. [5] (35.3%) that studied the prevalence of HLAR in 133 enterococcal strains obtained from different clinical samples at Alexandria Main University Hospital, Egypt. However, higher rates of HLAR enterococci (72%) were obtained by Padmasini et al. [11] in India. Another study by El Mahdy et al. [15] at Mansoura University Hospitals in Egypt showed also higher HLAR results (66.3%) in 80 enterococcal isolates recovered from urine samples. The present study revealed a high prevalence of HLSR strains (74%), although the clinical use of streptomycin for infections caused by enterococci has long been restricted due to intrinsic low-level resistance of the organism [11].
Previous studies by Padmasini et al. [11], Bhatt et al. [16], and Niu et al. [17] have reported that HLGR (42.7%, 65%, and 42.7%) was more common than HLSR (29.8%, 45%, and 27.4%) in all species of isolated enterococci respectively. On the contrary, there was nearly no difference between the prevalence rates of HLGR and HLSR among our studied Enterococcus species isolates. Similar finding was found in Turkey by Kurtgoz et al. [18].
HLAR was found to be more common in E. faecalis and E. faecium [19], on the other hand, Abamecha et al. [20] and Bhatt et al. [16] reported that HLAR was a common problem among E. faecium isolates only. A surveillance study that was conducted in 20 European countries had reported 32% and 22% HLGR and 41% and 49% HLSR among HLAR E. faecalis and E. faecium, respectively [19]. In the current study, HLGR is 70% and 100%, while HLSR is 76% and 67% among E. faecalis and E. faecium respectively. This emphasizes susceptibility differences within different enterococcal species.
In accordance with previous studies [5, 11, 17], the aac (6′)-Ie-aph (2′)-Ia gene (66.7%) and aph (3′)-IIIa (86.5%) were identified as the most common AME genes among HLGR and HLSR strains, respectively. Nevertheless, the prevalence of the previous two genes was variable among these former studies. Padmasini et al. [11] stated that the aac (6′)-Ie-aph (2′)-Ia was found in 68.4% of their HLAR enterococcal isolates which is comparable to our results, while 77.4% carried aph (3′)-IIIa gene. Niu et al. [17] and El Ghazawy et al. [5] revealed also higher rates of aac (6′)-Ie-aph (2″)-Ia gene being 89.3% and 95.7%, respectively, which reflects their finding of higher pervasiveness of HLGR in their isolates.
It is noteworthy that the newer aminoglycoside modifying genes aph (2′)-Ib, aph (2′)-Ic, and aph (2′)-Id that encode HLGR were not detected among the studied strains. This was in agreement with Padmasini et al. [11]. In contrast to our results, aph (2′)-Ic gene was detected by El-Ghazawi et al. [5].
Both aac (6′)-Ie-aph (2″)-Ia and aph (3′)-IIIa genes co-existed in isolated E. faecalis, E. faecium, and E. casseliflavus strains, which was in accordance with results obtained by Padmasini et al. [11] and El Mahdy et al. [15]. However, 9 out of the 50 of HLAR enterococcal strains did not carry any of the formerly tested genes. This may be due to the expression of genes other than genes analyzed in this study.