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Correspondence |
1 Área de Bioquímica y Biología Molecular, Departamento de Agricultura y Alimentación, Universidad de La Rioja, Logroño, Spain
2 Servicio de Microbiología, Hospital Universitario Central de Asturias, Oviedo, Spain
Correspondence
Carmen Torres
(carmen.torres{at}unirioja.es)
In the last few years, the emergence and wide dissemination of Escherichia coli strains showing resistance to broad-spectrum cephalosporins and monobactams, due to the production of extended-spectrum β-lactamases (ESBLs), has been reported. In the 1990s, early publications on ESBLs reported blaSHV or blaTEM variants, which were especially detected in Klebsiella isolates recovered in intensive care units. The situation has been changing in the last few years and CTX-M β-lactamases are an emerging mechanism of resistance, mainly among E. coli isolates (Cantón et al., 2008; Rossolini et al., 2008). More than 65 different CTX-M β-lactamases belonging to six different groups have been reported to date (Rossolini et al., 2008).
Different genetic environments might be involved in the mobilization of blaCTX-M genes. The insertion sequence ISEcp1, which is known to mobilize the sequences located at its right-end extremity, has been found upstream of a wide variety of blaCTX-M genes (Poirel et al., 2008). In addition, IS26, IS10, IS5 and IS903 have been detected surrounding different blaCTX-M genes (Eckert et al., 2006; Poirel et al., 2008; Cartelle et al., 2004; Lartigue et al., 2004). The blaCTX-M genes have also been identified in complex sul1-type class 1 integrons such as In60 (Sabaté et al., 2002; Riaño et al., 2006; Novais et al., 2006), or associated with a phage-related element (Oliver et al., 2005). The production of plasmidic class C β-lactamases, such as CMY enzymes, or the overproduction of the chromosomal AmpC β-lactamase (Briñas et al., 2005; Caroff et al., 2000; Mulvey et al., 2005; Stapleton et al., 1999) can also be associated with broad-spectrum cephalosporin-resistance in E. coli.
The objective of this work was to carry out β-lactamase characterization for all broad-spectrum cephalosporin-resistant E. coli isolates recovered during a 1 year period (2003–2004) in a Spanish hospital, in order to determine the genetic environment of the blaCTX-M genes, and their possible inclusion in integrons. The results obtained in this study were compared with previous data obtained for E. coli isolates recovered by our group in the same hospital in 2002 (Briñas et al., 2005), in order to track the evolution of ESBLs and other mechanisms of broad-spectrum-cephalosporin resistance.
Among the 1376 E. coli isolates recovered from patients of the Hospital Universitario Central de Asturias (Oviedo, Spain) during a 1 year period (April 2003 to March 2004), 61 isolates (4.4 %) showed a MIC value for cefotaxime (CTX) and/or ceftazidime (CAZ) 
2 µg ml–1, and were included in this study. The origins (and numbers) of the isolates were as follows: urine (37), exudates and surgical wounds (10), blood (4), tracheobronchial aspirate (5), pus (3), bile (1) and cephalorachidian liquid (1).
Antibiotic susceptibility to amoxicillin, amoxicillin/clavulanic acid (AMC), cephalothin, cefoxitin, CTX, CAZ, cefepime, imipenem, nalidixic acid, ciprofloxacin, gentamicin, amikacin, tobramycin, fosfomycin and trimethoprim/sulfamethoxazole (SXT) was studied by agar dilution and/or the disc diffusion method (CLSI, 2007). E. coli ATCC 25922 was used as a quality control strain. To determine ESBL production, a double-disc synergy test (Jarlier et al., 1988) between AMC and CTX or CAZ was performed, and this screening test was positive in 56 isolates and negative in the remaining 5 isolates, representing 4.1 and 0.4 %, respectively, of the total 1376 E. coli isolates. In a previous work carried out by our group in the same hospital in 2002, the prevalence of this type of isolate was 1.4 and 0.6 %, respectively, among 1700 E. coli isolates (Briñas et al., 2005).
The presence of genes encoding TEM, SHV, CTX-M and CMY type β-lactamases was studied in the strains by specific PCRs (Jouini et al., 2007; Bertrand et al., 2006; Stapleton et al., 1999). All obtained amplicons were sequenced on both strands, and sequences were compared with those included in the GenBank database and on the Lahey Clinic website (http://www.lahey.org/Studies/) in order to identify the β-lactamase-encoding genes. The MICs of different β-lactams and the type of β-lactamases detected in the 56 ESBL-positive isolates are shown in Table 1
, being the following β-lactamase-encoding genes (the numbers of isolates are shown in parenthesis): blaCTX-M-14 (29), blaCTX-M-9 (9), blaCTX-M-15 (1), blaCTX-M-32 (2), blaSHV-12 (12), blaSHV-2 (1), blaTEM-52 (1) and a new blaSHV gene (1).
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). This new SHV variant has been named as SHV-102 and included in the GenBank database with the accession number EU024485. The Gly238Ala substitution was previously described in SHV-13, SHV-18 and SHV-29 ESBLs (in addition to amino acid changes at other positions), and one unique amino acid change at this position (Gly238Ser) has been reported to hydrolyse CTX (Huletsky et al., 1993). It is interesting to note that 41 of our 56 ESBL-positive E. coli isolates (73.2 %) harboured a β-lactamase of CTX-M class, 14 isolates (25 %) a β-lactamase of SHV class (with 3 different molecular variants) and 1 isolate (1.8 %) a β-lactamase of TEM class. A gene encoding a β-lactamase of the CTX-M-9 group (blaCTX-M-14 or blaCTX-M-9) was detected in 38 of the 41 blaCTX-M-containing strains (92.6 %). If these results are compared with the previous ones obtained in the same hospital (Briñas et al., 2005), the evolution shows a diversification of the ESBLs of CTX-M and SHV types. It also seems that SHV-type ESBLs are increasing in prevalence (12.5 % in 2002 versus 25 % in 2003–2004), mainly SHV-12 enzymes, and that TEM-type ESBLs are decreasing in importance (8.3 % in 2002 versus 1.8 % in 2003–2004). A similar tendency has been observed in other studies (Brasme et al., 2007; Valverde et al., 2004; Paterson et al., 2003). It would be interesting to track the evolution of these genes in the following years to detect whether this tendency is maintained.
The percentages of resistance to non-β-lactam antibiotics for our ESBL-positive isolates were as follows: nalidixic acid (64 %), ciprofloxacin (32 %), gentamicin (18 %), tobramycin (12.5 %), amikacin (9 %), SXT (66 %) and fosfomycin (0 %). High percentages of resistance to quinolones were observed among our ESBL isolates, and similar observations have been reported by other authors (Morosini et al., 2006).
The clonal relationship among the 41 blaCTX-M-containing isolates was studied by PFGE, using XbaI as restriction enzyme (Sáenz et al., 2004), and patterns were analysed according to reported criteria (Tenover et al., 1995). A total of 37 unrelated PFGE patterns were identified among the blaCTX-M-containing E. coli isolates, and 1 closely related or indistinguishable PFGE pattern was found in 4 of the CTX-M-positive isolates (Table 2). This high clonal diversity observed among our blaCTX-M-containing E. coli isolates is coincident with similar results obtained in other studies (Velasco et al., 2007; Hernández et al., 2005, Ho et al., 2007; Briñas et al., 2005), indicating that the wide dissemination of the blaCTX-M genes among E. coli isolates is mainly due to the horizontal dissemination of resistance determinants and not to dissemination of resistant clones. Nevertheless, the clonal dissemination of CTX-M-15-producing E. coli isolates within the hospital and community has been reported, and very recently, the intercontinental emergence of E. coli clone O25:H4-ST131 producing CTX-M-15 has been reported (Nicolas-Chanoine et al., 2008).
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, all nine blaCTX-M-9-containing isolates showed this gene inside the In60 integron, which includes the ISCR1 upstream of the blaCTX-M-9 gene and orf3-like and orf1005 downstream of this bla gene. The In60 integron has been described by Sabaté et al. (2002), and was detected later by other authors (Novais et al., 2006; Riaño et al., 2006). All 29 E. coli isolates that contained the blaCTX-M-14 gene presented the ISEcp1 upstream and IS903 downstream of this blaCTX-M-14 gene. The whole IS903 sequence was obtained in eight of the isolates, whereas a truncated IS903 was present in the remaining ones; both structures have been described by Eckert et al. (2006).
The genetic environment of the blaCTX-M-15 and blaCTX-M-32 genes detected in the three remaining blaCTX-M-containing isolates included the ISEcp1 and the ISEcp1/IS5 upstream of blaCTX-M-15 and blaCTX-M-32, respectively, and the orf477 downstream of both bla genes, as has been detected by others (Eckert et al., 2006; Cartelle et al., 2004).
The detection of class 1 integrons, as well as the characterization of the gene cassettes included in their variable regions, were studied in all 41 blaCTX-M-containing E. coli isolates by PCR and sequencing. The results are presented in Table 2. The intI1 and the variable region of class 1 integrons were detected in 23 of these isolates. Two of them presented the intI1 gene interrupted at the 3' end by the insertion of the IS26 element (data not shown). Some class 1 integrons lacking a functional integrase have been described already (i.e. GenBank accession numbers AF205943 and AJ870926), although none of them are identical to our integron that presented the following composition: IS26+intI1
+dfrA1+aadA1+qacE1+sul1.
A wide variety of gene cassette arrangements were detected in the variable region of the class 1 integrons in these 23 isolates: dfrA16+aadA2 inside In60 (9 isolates), dfrA1+aadA1 (7 isolates), dfrA17+aadA5 (4 isolates), aadA1 (2 isolates) and aadB+aadA1+cmlA1 (1 isolate). In addition, the gene encoding the class 1 integrase was found in three additional isolates in which no variable region could be identified (Table 2). The absence of the 3' conserved region (qacE1+sul1 genes) in these strains could explain the negative result, as has been described in other studies (Sáenz et al., 2004). The class 1 integrons detected in our study contained gene cassettes that confer resistance to streptomycin (aadA1, aadA2, aadA5) and/or trimethoprim (dfrA1, dfrA16, dfrA17). The unusual gene cassette combination aadB+aadA1+cmlA1 has been detected in pathogenic E. coli isolates from pigs, calves and humans (Bischoff et al., 2005; Chang et al., 2000; Du et al., 2005). The aadB gene confers resistance to gentamicin and the cmlA1 gene provides resistance to chloramphenicol.
In addition, the presence of sul1, sul2 and sul3 genes, associated with sulfamethoxazole resistance, was also analysed by PCR in all blaCTX-M positive isolates (Sáenz et al., 2004), and 32 of the 41 blaCTX-M-containing E. coli isolates harboured the following genes: sul1 (24 isolates), sul2 (19 isolates) and sul3 (9 isolates), and more than one sul gene were detected in 18 of them (56 %).
In the five broad-spectrum-cephalosporin-resistant and ESBL-negative isolates found in this work, the promoter and attenuator region of the chromosomal ampC gene was amplified by PCR (Briñas et al., 2005), sequenced, and compared with the same region of the E. coli K12 ampC gene. Mutations at positions –42 (C
T), –18 (GA), –1 (CT) and +58 (CT) were detected in two of the isolates (Table 1), and resistance could be associated with hyperproduction of chromosomal AmpC β-lactamase, as reported by Caroff et al. (2000) and Mulvey et al. (2005). The plasmidic CMY-2 β-lactamase was identified in another E. coli isolate and no mechanism of resistance could be found in the remaining two ESBL-negative E. coli isolates (Table 1).
In summary, our results show an important increase in the prevalence of ESBLs among the E. coli isolates recovered in 2003–2004 in the studied hospital, with respect to a previous study carried out in 2002, the most prevalent ones being the ESBLs of the CTX-M class, especially those of the CTX-M-9 group (CTX-M-15 and CTX-M-32 were also detected, but they were a minority). In addition, it seems that ESBLs of the SHV-type are growing in importance, representing 25 % of the total ESBLs detected, and a diversification of the SHV variants is occurring, which includes the identification of a new SHV-102 variant. More studies should be carried out in the future to track the evolution in time of ESBLs in isolates from different environments, countries and continents.
ACKNOWLEDGEMENTS
This work was partially supported by the project SAF2006-14207-C02-01 from the Ministry of Education and Science of Spain. L. V. was supported by a fellowship of the Spanish Ministry of Education and Science (SAF2006-14207-C02-01).
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