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Sensibilidad de enterobacterias de origen animal y agua frente a antibioticos betalactamicos.

Beta-lactam resistance in enterobacteria isolated from animal and water


The most common mechanism by which bacteria are resistant to antibiotics is by producing enzymes that inactivate the drugs (1) . [beta]-lactam antibiotics (penicillins and cephalosporins) can be inactivated by enzymes known as [beta]-lactamases (1, 4). Hundreds of [beta]-lactamases have been described; they can be both plasmid or chromosomic encoded, and have varying degrees of activity against the different [beta]-lactam antibiotics (4). Many bacteria produce multiple [beta]-lactamases. In response to the proliferation and spread of [beta]-lactamases, the pharmaceutical industry has developed some [beta]-lactam antibiotics that are more resistant to hydrolysis by these enzymes (2). In addition, some combination drugs have been produced which contain both a [beta]-lactam antibiotic and a [beta]-lactamase inhibitor; the inhibitor has high affinity for the [beta]-lactamase enzyme, irreversibly binds to it, and thereby preserves the activity of the [beta]-lactam antibiotic (10). About one-tenth of isolates of gram-negative pathogens seem to produce extended-spectrum beta-lactamases (ESBL) (18).

The phenotipic detection of ESBL are based in the inhibition of the majority of these enzymes by clavulanic acid and the utilization of 3[degrees] and 4[degrees] generation cephalosporins and aztreonam as indicators (18). The double disc synergy proof consist to situate a clavulanic-amoxicillin disc next to [beta]-lactam antibiotic discs as indicators. The ESBL production is demonstrated by enlarging the inhibition halo of any indicator by the clavulanic acid action. The discs with inhibitors is one of the recommended methods of the Clinical Laboratory and Standards Institute (3). This method consists in comparing a 3[degrees] or 4[degrees] generation cephalosporin-inhibition zone alone or with clavulanic acid. The activity increase of cephalosporin in presence of clavulanic acid indicates ESBL production. ESBL genes are frequently in the same plasmids that codifies aminoglucosid and sulfonamide resistance. Some enterobacteria species posses changes that add quinolone resistance so that means multiresistance (10).

A mechanism founded in some species naturally transformed, had been described as [beta]-lactam resistant, where penicillin binding proteins (PBP) were altered, presumably by transformation, with reduced affinity to [beta]-lactam antibiotics (7, 15, 16). The resistance to these, in the majority of Gram negative species and some Gram positive, is due to the presence of a [beta]-lactamase, which modifies the antibiotic, avoiding its fixation to PBPs (18). Drug efflux systems, pump out a broad range of chemically and structurally unrelated compounds from bacteria, in an energy-dependent manner, without drug alteration or degradation (13). Although drug efflux pumps are found in Gram-negative and Gram-positive bacteria, efflux mediated resistance in Gram-negative bacteria is a more complex problem due to the molecular architecture of the cell envelope (6, 9). As a consequence, drug resistance in many cases is attributable to synergy between reduced drug intake (mainly due to low outer membrane permeability) and active drug export (via efflux pumps) (11).

Along with the steady increase of nosocomial infection rates in veterinary clinics, particular attention has recently been drawn to the genetic background of multi-resistant strains, resulting in the identification of certain genetic lineages which frequently appear in both, human and animal samples: extended-spectrum [beta]-lactamases (ESBL)-producing Enterobacteriaceae (14). These sequence types (ST), include the pandemic ST131 for ESBL-producing E. coli (18).

The isolation of opportunistic Proteus mirabilis and Proteus vulgaris is very frequent in clinical samples from canine otitis at the Bacteriologic and Mycologic Diagnostic Laboratory of the Microbiology Area of the Veterinary Science Unit at National North East University of Corrientes, Argentina. In eight years (from 2006 to present) over 339 pathological samples, in 69 (20,3%) enterobacteria were isolated, from these, 44 strains (63,7%) were Proteus spp, belonging to mirabilis (n=33, 75%) and vulgaris (n=11, 25%) species, followed by Enterobacter spp (n=14, 20,3%), Klebsiella spp (n=7, 10,1%) and Escherichia coli (n=6, 8,7%). Other isolates were 38 strains, corresponding 12 to Enterobacter spp., 8 to E. coli and Proteus spp. respectively, 6 to Klebsiella spp. and 4 to Citrobacter spp from 21 purulent exudates, 2 nasal, conjuntival and vaginal swabs each one, 2 faecal cultures and 2 urocultures. As we do not know the resistance of enterobacteriae from water we included those isolated from non-chlorinated water of rural area.

As the serine-[beta]-lactamases, AmpCs, are induced and occur naturally in Enterobacter spp., C. freundii and S. marcescens (8, 12), they were excluded of this study owing their intrinsic antimicrobial resistance (inherent or innate, not acquired) which is reflected in wild type antimicrobial patterns of all or almost all representative species (3).

Owing that enterobacteria isolated from these pathological process presented resistance to different [beta]-lactam antibiotics, the aim of the present work was to try phenotypically determined the origin of this, to establish susceptibility patterns of each genre in order to know the best options for treatment.


Thirty seven strains of enterobacteria, 29 from clinical samples (Table 1) and 8 from non-chlorinated water of rural areas of Corrientes Province, at North East of Argentina: E. coli (n=5), K. pneumoniae (n=2) and K. oxytoca (n=1), were studied for their susceptibility to [beta]-lactam antibiotics using antibiotic-diffusion on Mueller-Hinton agar by Kirby-Bauer antibiogram (3).

Antibiotic discs (Britania[R]) of ampicillin (Ampi 10 [micro]g), cefotaxime (Cftx 30 [micro]g), cefepime (Fep 30 [micro]g), piperacillin (Pip 100 [micro]g) and with [beta]-lactam inhibitors: clavulanic acid-amoxicillin (CAM 20/10 [micro]g), sulbactam-cefoperazone (S-Cfpz 75/30 [micro]g) and tazobactampiperacillin (TAZ 100/10 [micro]g) were used. To detect extended spectrum [beta]-lactamases (ESBL) strains which hidrolyzed 3a generation cephalosporins (Cftx, ceftazidime Caz 30 [micro]g) as well as the monobactam aztreonam (Azt 30 [micro]g) and those inhibited by CAM but not by cefoxitin (Fox 30 [micro]g) were observed; boronic acid discs (Bor) were also used because its hability to inhibit plasmidic AmpC serin [beta]-lactamases (AmpCp) with 3[degrees] generation Cfp as indicators (17).

The strains were also analysed to phenotypically determine the presence of metallo-[beta]-lactamase (MBL): monodiscs of the carbapenems imipenem (Im, 10 [micro]g) and meropenem (Mer, 10 [micro]g) with one of ethilendiaminotetraacetic acid (EDTA, 1 [micro]mol) acting as MBL inhibitor were used according the agar diffusion method (6). The test is positive when there is synergism between EDTA and carbapenem discs.






All strains of Proteus mirabilis (7/7, 100%) and 5/16 (31,2%) of E. coli were resistant to ampicillin. With the aggregate of clavulanic acid to amoxicyllin (CAM) they turned susceptible in 71,4 and 87,5 % respectively.

Four strains of P. mirabilis were simultaneously resistant to Ampi and Cftx and susceptible to CAM and S-Cfpz; two strains of K. pneumoniae and one E. coli resistant to Pip were sensible to TAZ. With respect to E. coli, 3/16 were resistant to Ampi and susceptibly to CAM and presented simultaneous resistance to Cftx and Azt. There was not observed enlarged inhibition halo for ESBL phenotypic detection in these strains.

Cefoxitin resistance was present in K. pneumoniae (3/11, 27,2%) and P. mirabilis (3/7, 42,8%) that naturally lack the AmpC gene. Metallo [beta]-lactamases were not detected in these strains although resistance to imipenem, was observed in 9 of them: 3 K. pneumoniae, 3 E. coli and 3 P. mirabilis, all of animal origin.


Cefoxitin resistance found in this work in bacterias like K. pneumoniae and P. mirabilis that naturally lack the AmpC gene, may be a sign of the presence of plasmidic AmpC of epidemiological importance because its facility to horizontal dissemination. One characteristic of chromosomic type AmpC [beta]-lactamases is that they have no effect on 4[degrees] generation cephalosporins neither on carbapenems so they are the [beta]-lactam antibiotic of therapeutic election.

As there was not detected ESBL, it is concluded that the mechanisms of resistance may be produced by means of altered PBP that reduce affinity to [beta]-lactam antibiotics as well as the synergy between reduced drug intake and active efflux pumps.

As the phenotypic screening was also negative for detecting metallo-[beta]-lactamases (MBLs) producing isolates, the resistance to imipenem observed in nine strains, was more likely due to a decrease in the expression of an outer membrane protein channel for imipenem (11). Owing that MBLs do not efficiently hydrolyze aztreonam, its sensibility would be a good predictor of the enzyme presence in resistant bacteria to imipenem and meropenem (5). This was not the case in this work, where all the strains susceptible to aztreonam were also susceptibly to meropenem.

We agree with authors that interdisciplinary approaches including human and veterinary experts should be implemented to develop reliable investigation procedures with respect to the current reality of animal owners and their pets (18). Additionally, consequent basic hygienic measures, prudent use of antimicrobials in companion animals and efforts regarding implementation of antibiotic stewardships should be fostered.


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Cicuta, M.E. [1]; Roibon, W.R. [1]; Barcelo, M.C. [1]; Arzu, O.R. [2]; Amable, V.I. [1]

Catedras de Microbiologia [1] y Bromatologia [2], Facultad de Ciencias Veterinarias, Universidad Nacional del Nordeste, Sargento Cabral 2139, Corrientes (3400), Argentina. Tel/Fax 54-379-4425753. E-mail:

Recibido: 30 octubre 2013 / Aceptado: 19 diciembre 2013
Table 1. Detail of 29 strains of Enterobacteria of animal origin.

No. bacterial strain       origin                   reference

 1 Klebsiella pneumoniae   canine otitis            Ex O 1031
 2 Klebsiella pneumoniae   rabbit nasal exudate     As 163
 3 Klebsiella pneumoniae   mare vaginal discharge   Ex V 129
 4 Klebsiella pneumoniae   cat nasal discharge      Ex N 86
 5 Klebsiella pneumoniae   canine piodermitis       Ex P 183
 6 Klebsiella pneumoniae   canine piodermitis       Ex P 190
 7 Klebsiella pneumoniae   canine piodermitis       Ex P 201
 8 Klebsiella pneumoniae   canine piodermitis       Ex P 204
 9 Klebsiella pneumoniae   canine uroculture        O 47
10 Klebsiella oxytoca      equine dermal ulcer      U 163

 1 Escherichia coli        canine otitis            Ex O 1056
 2 Escherichia coli        canine otitis            Ex O 1060
 3 Escherichia coli        minced bovine meat       C 441
 4 Escherichia coli        minced bovine meat       C 448
 5 Escherichia coli        equine dermal ulcer      U 162
 6 Escherichia coli        canine nasal discharge   Ex N 91
 7 Escherichia coli        calf faecal swab         MF 21
 8 Escherichia coli        calf faecal swab         MF 28
 9 Escherichia coli        calf faecal swab         MF 29
10 Escherichia coli        calf faecal swab         MF 30
11 Escherichia coli        calf faecal swab         MF 31
12 Escherichia coli        deer liver               As 165

 1 Proteus mirabilis       canine piodermitis       ExP 177
 2 Proteus mirabilis       canine otitis            Ex O 1011
 3 Proteus mirabilis       canine otitis            Ex O 1030
 4 Proteus mirabilis       canine otitis            Ex O 1045
 5 Proteus mirabilis       equine dermal ulcer      U 160
 6 Proteus mirabilis       equine dermal ulcer      U 162
 7 Proteus mirabilis       ant-bear fecal swab      AS 164
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Author:Cicuta, M.E.; Roibon, W.R.; Barcelo, M.C.; Arzu, O.R.; Amable, V.I.
Publication:Revista Veterinaria
Date:Jul 1, 2014
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