The CLSI disk diffusion ESBL confirmatory test proved suitable for the identification of K. pneumoniae isolates included in this study, correctly identifying 41 (78.85%) isolates as possessing β-lactamases capable of hydrolysing oximino-cephalosporins. Although it was not the intention of this study to evaluate the effectiveness of the Microscan WalkAway-96 System, the sensitivity being reported here is much less than that reported by Wiegand et al. [15] of 84 and 87% by Vespero et al. [16]. However, this 21.15% (11/52) “false positive” rate should be interpreted with caution as well as interest. In a SENTRY report authored by Bell et al. [17], they found that 20.3% of screen-positive isolates failed to show clavulanate synergy, and, subsequently, 75% of these nonconforming results were due to the presence of a plasmid-borne AmpC enzyme of the CIT or DHA type. Munier et al. [18] also found that 70% of ESBL screen-positive isolates (which were characterised by E. coli, K. pneumoniae, K. oxytoca, and Proteus mirabilis) were actually producing an AmpC β-lactamase, while only 13% represented true ESBL producers. Although this study did assess whether isolates that produced negative confirmatory results possessed a gene that coded for an AmpC β-lactamase (data not shown), all of the isolates returned negative results. However, this finding does not rule out the possibility of these isolates harbouring other AmpC varieties, and further investigation is warranted.
Another possible reason that may be posited for the “high” number of false positives involves the influence of the inoculum effect. In their clinical update paper, Patterson and Bonomo highlight that in vitro, MICs of cephalosporins may rise as the inoculum of ESBL-producing organism increases [5]. This was further substantiated by Thauvin-Eliopoulos et al. [19] who showed that the cefotaxime MIC for a K. pneumoniae strain harbouring TEM-26 increased from 0.25 μg/ml at an inoculum of 105 CFU/ml to 64 μg/ml at an inoculum of 107 CFU/ml. Similarly, Bedenic et al. [20] found that SHV harbouring klebsiellae were more resistant to cephalosporin agents when the inoculum size was higher. This reason may certainly be applicable in the case of this study especially given that eight of nine negative confirmatory test isolates were identified as possessing a SHV-type β-lactamase gene when examined with a molecular assay.
It is also worth noting that even though there was PCR amplification of TEM and SHV genes in most of the isolates, without sequencing it cannot be determined whether these genes contributed to mediating extended spectrum beta-lactam resistance. Sequencing allows for the differentiation of the non-ESBL genes (e.g., TEM-1, -2, and SHV-1) from the ESBL variants (e.g., TEM-3 and SHV-2) [21].
The distribution of the three groups of ESBL-genes for K. pneumoniae identified in this study was different from what was reported at this institution by Akpaka et al. [2]. That study reported that there were 84.3% bla
TEM, 34.5% bla
SHV and 58.8% bla
CTX-M of the ESBL genes present in K. pneumoniae isolates recovered at the institution (EWMSC) over a 3-year period. In comparison, this study found lower rates for bla
TEM (59%; 39/66) and bla
CTX-M (46.9%; 31/66, bla
CTX-M1 plus bla
CTX-M2), and a higher rate for bla
SHV (84.8%; 56/66). Most noteworthy is that 37.8% (25/66) of the K. pneumoniae isolates possessed all three β-lactamase genes. This suggests that one or more of these β-lactamase genes may have been acquired from transferrable plasmids, however, a conjugative assay was not performed at the time the study was conducted to confirm this. Moreover, this finding increases the likelihood that other genes such as plasmid-mediated fluoroquinolone and aminoglycoside resistance genes may also be co-transferred, thereby contributing to the dissemination of multidrug resistance mechanisms [22].
The emergence and spread of CTX-M continues to be well documented and reported across Latin America and the Caribbean [2, 9, 23, 24]. In this study there was widespread distribution of β-lactamases belonging to the CTX-M-1 group, and one instance of an isolate with a β-lactamase from the CTX-M-2 group- the first such account of this particular enzyme in K. pneumoniae from a Caribbean territory. This simultaneous production of both cefotaximase and ceftazidimase poses a serious problem to the characterization of resistance by clinical laboratories since these enzymes confer a higher level of resistance to oxyimino-cephalosporinases [25].
For RAPD typing, isolates that possessed two or more β-lactamase genes were chosen with the majority of isolates being those recovered in 2015. RAPD-PCR was chosen because it is a rapid and simple method which when optimized has proven competitive with the gold standard of pulse field gel electrophoresis [26, 27]. RAPD1, also referred to as Primer 640 by other authors [13, 26], proved to be optimal for DNA fingerprinting since it allowed the clear distinction of DNA banding patterns for all of the isolates tested. The resulting RAPD profiles for this select group of isolates showed that there was diversity among those isolated during 2008–2010 and those recovered in 2015. However, it was found that K. pneumoniae isolates with the same genotype possessed three or more extended-spectrum β-lactamase genes. This seems to suggest that these strains may have been epidemic in the hospital environment, but due to the lack of patient information this fact could not be proven with certainty.
This study was not without some limitations. The loss of transferrable genetic elements, i.e. plasmids, in stored K. pneumoniae isolates from 2008 to 2010 could not be accounted for, and, therefore, genes contributing to resistance might have been lost as has been highlighted by previous authors [5]. Secondly, RAPD-PCR was only conducted on a small subset of the isolates recovered due to inadequate resources for DNA extraction and purification.
