Ability of newer beta-lactam antibiotics to induce beta-lactamase production in Enterobacter cloacae

Role of AmpC-Inducing Genes in Modulating Other Serine Beta-Lactamases in Escherichia coli

The consistently mutating bacterial genotypes appear to have accelerated the global challenge with antimicrobial resistance (AMR); it is therefore timely to investigate certain less-explored fields of targeting AMR mechanisms in bacterial pathogens. One of such areas is beta-lactamase (BLA) induction that can provide us with a collection of prospective therapeutic targets. The key genes (ampD, ampE and ampG) to which the AmpC induction mechanism is linked are also involved in regulating the production of fragmented muropeptides generated during cell-wall peptidoglycan recycling. Although the involvement of these genes in inducing class C BLAs is apparent, their effect on serine beta-lactamase (serine-BLA) induction is little known. Here, by using ∆ampD and ∆ampE mutants of E. coli, we attempted to elucidate the effects of ampD and ampE on the expression of serine-BLAs originating from Enterobacteriaceae, viz., CTX-M-15, TEM-1 and OXA-2. Results show that cefotaxime is the preferred inducer for CTX-M-15 and amoxicillin for TEM-1, whereas oxacillin for OXA-2. Surprisingly, exogenous BLA expressions are elevated in ∆ampD and ∆ampE mutants but do not always alter their beta-lactam susceptibility. Moreover, the beta-lactam resistance is increased upon in trans expression of ampD, whereas the same is decreased upon ampE expression, indicating a differential effect of ampD and ampE overexpression. In a nutshell, depending on the BLA, AmpD amidase moderately facilitates a varying level of serine-BLA expression whereas AmpE transporter acts likely as a negative regulator of serine-BLA.

Multi-drug resistant pathogenic bacteria in the gut of young children in Bangladesh

BACKGROUND

The gut of human harbors diverse commensal microbiota performing an array of beneficial role for the hosts. In the present study, the major commensal gut bacteria isolated by culturing methods from 15 children of moderate income families, aged between 10 and 24 months, were studied for their response to different antibiotics, and the molecular basis of drug resistance.

RESULTS

Of 122 bacterial colonies primarily selected from Luria-Bertani agar, bacterial genera confirmed by analytical profile index (API) 20E^® system included Escherichia as the predominant (52%) organism, followed by Enterobacter (16%), Pseudomonas (12%), Klebsiella (6%), Pantoea (6%), Vibrio (3%), and Citrobacter (3%); while Aeromonas and Raoultella were identified as the infrequently occurring genera. An estimated 11 and 22% of the E. coli isolates carried virulence marker genes stx-2 and eae, respectively. Antimicrobial susceptibility assay revealed 78% of the gut bacteria to be multidrug resistant (MDR) with highest resistance to erythromycin (96%), followed by ampicillin (63%), tetracycline (59%), azithromycin (53%), sulfamethoxazole-trimethoprim (43%), cefixime (39%), and ceftriaxone (33%). PCR assay results revealed 56% of the gut bacteria to possess gene cassette Class 1 integron; while 8, 17.5 and 6% of the strains carried tetracycline resistance-related genes tetA, tetB, and tetD, respectively. The macrolide (erythromycin and azithromycin) resistance marker genes mphA, ereB, and ermB were found in 28, 3 and 5% of bacterial isolates, respectively; while 26, 12, 17, 32, 7, 4 and 3% of the MDR bacterial isolates carried the extended spectrum β-lactamase (ESBL)-related genes e.g., bla_TEM, bla_SHV, bla_CMY-9, bla_CTX-M1, bla_CTX-M2, bla_CMY-2 and bla_OXA respectively. Majority of the MDR gut bacteria harbored large plasmids [e.g., 140 MDa (43%), 105 MDa (30%), 90 MDa (14%)] carrying invasion and related antibiotic resistance marker genes.

CONCLUSIONS

Our results suggest gut of young Bangladeshi children to be an important reservoir for multi-drug resistant pathogenic bacteria carrying ESBL related genes.

Phenotypic characterization of ESBL producing Enterobacter cloacae among children

Objective: The emergence of ESBL producing Enterobacter cloacae in clinical isolates is posing a serious threat for treating nosocomial infections. The aim of the study was to determine the frequency of extended spectrum β-lactamase (ESBL) producing Enterobacter cloacae and to compare the phenotypic methods used for the characterization of ESBL producing strains.

Methodology: This cross sectional observational study was conducted during April 2011 to March 2012 at Microbiology department of The Children’s Hospital and Institute of Child Health, Lahore. A total number of 20,257 various clinical samples were analyzed during the study period. Enterobacter cloacae were identified using API 20E system and ESBL detection was carried out using double-disk synergy test (DDST) and CLSI confirmatory test.

Results:Enterobacter cloacae were isolated from 221 samples, out of which 33 (14.93%) were ESBL producers and 188 (85.07%) were non-ESBL producers. The gender distribution of ESBL producing Enterobacter cloacae was 21 (63.6%) in males and 12 (36.4%) in females. Highest frequency (63%) of ESBL producing Enterobacter cloacae was detected in blood samples. Comparison of DDST and CLSI confirmatory test showed that 25 (75.75%) isolates were characterized by DDST and 33 (100%) using CLSI confirmatory test.

Conclusion: The present study shows moderately high frequency of ESBL producing Enterobacter cloacae among children. DDST was found to be less efficient in ESBL detection as compared to CLSI confirmatory test.

Occurrence of virulence-associated properties in Enterobacter cloacae

Enterobacter cloacae is not a primary human pathogen but has been considered to be an important cause of nosocomial infections. Even so, there are almost no reports on its ability to produce recognized virulence-associated properties. In this study, we show that most of the E. cloacae strains examined were resistant to serum bactericidal activity and were able to produce aerobactin and mannose-sensitive hemagglutinin, and all of them could adhere to and invade HEp-2 cells. Since E. cloacae is part of the normal intestinal floras of many individuals, we believe that infectious disease due to endogenous E. cloacae might be a result of both host predisposing factors and the bacterial virulence determinants that we have detected in this survey.

Resistance characteristics of blood culture isolates of Enterobacter cloacae with special reference to beta-lactamases and relation to preceding antimicrobial therapy

Resistance characteristics of 53 blood culture isolates of E. cloacae were examined and correlated with antimicrobial treatment preceding bacteraemia. Resistance patterns of 22 antimicrobial agents, presence of resistant mutants, and inducibility of beta-lactamase were investigated; furthermore, population analysis and investigation of beta-lactamase production of selected isolates were performed. Thirty-two isolates (60%) were resistant to cephalothin and/or cefoxitin and/or ampicillin, and 14 isolates (26%) had further resistance characteristics, 7 of the 14 being resistant to non-beta-lactam antibiotics. All ampicillin-susceptible and 76% of cefotaxime-susceptible isolates had resistant mutants in the zone of inhibition when high inoculum was used. All isolates investigated had inducible chromosomal beta-lactamases, and, in addition, two isolates had an enzyme corresponding with TEM-1. Correlation of resistance patterns and antimicrobial treatment preceding bacteraemia showed that treatment with a third-generation cephalosporin was associated with beta-lactam multiresistance. In conclusion, susceptibility testing of beta-lactam antibiotics of Enterobacter must be interpreted with caution and monotherapy with an extended-spectrum cephalosporin should be avoided unless presence of resistant mutants and inducibility of beta-lactamase can be excluded.

Importance of beta-lactamase induction

Induction of an enzyme is a temporary phenomenon in which the rate of enzyme synthesis is greatly increased in response to the presence of an inducer in the environment. Induction of beta-lactamase synthesis is important for the resistance of staphylococci to penicillins since the drug both induces synthesis of the enzyme and is hydrolysed by it. Similarly, some compounds both strongly induce and are hydrolysed by the chromosomally-determined beta-lactamases of gram-negative bacilli (e.g. amoxicillin and cefoxitin for Enterobacter cloacae). Other compounds (e.g. piperacillin and cefotaxime) although labile are poor inducers, so that in the case of these drugs the phenomenon of induction is not important but the presence of the enzyme is, since resistant mutants with genetically derepressed synthesis of the enzyme can emerge. Induction can also be important when a strong inducer is present with a poor inducer and antagonises the activity of the latter.

Induction of beta-lactamase production in Pseudomonas aeruginosa biofilm

Imipenem induced high levels of beta-lactamase production in Pseudomonas aeruginosa biofilms. Piperacillin also induced beta-lactamase production in these biofilms but to a lesser degree. The combination of beta-lactamase production with other protective properties of the biofilm mode of growth could be a major reason for the persistence of this sessile bacterium in chronic infections.

Reevaluation of the factors involved in the efficacy of new beta-lactams against Enterobacter cloacae

The roles of outer membrane permeability, beta-lactamase stability, and inhibition of penicillin-binding proteins in the activity of new beta-lactams against Enterobacter cloacae were reappraised by using several methodological improvements. Outer membrane permeability in intact cells was determined by using a high-pressure liquid chromatography (HPLC)-based technique that avoided certain possible artifacts of the traditional methods. Vmax values were calculated from the numbers of enzyme molecules produced per cell and from catalytic constant (Kcat) values, which were obtained with purified beta-lactamase. Minimal periplasmic antibiotic concentrations needed to inhibit bacterial cell wall synthesis were estimated from the Zimmermann-Rosselet equation. All the beta-lactams tested formed relatively stable complexes with purified beta-lactamase. The antibiotics that exhibited low affinity for beta-lactamase apparently needed higher periplasmic concentrations to inhibit cell wall synthesis, suggesting a possible correlation between the affinity of beta-lactamase and the affinity of penicillin-binding proteins for the new beta-lactams. By using these estimates of outer membrane permeability, beta-lactamase hydrolysis, and cell wall-inhibiting concentrations, MIC could be theoretically predicted to within 1 dilution for five beta-lactams in three isogenic E. cloacae strains with differences in antibiotic susceptibility due to different porin or beta-lactamase contents.

In vitro activity of combinations of beta-lactam antibiotics with beta-lactamase inhibitors against cephalosporinase-producing bacteria

Combinations of different beta-lactam antibiotics, including cefotaxime, with three beta-lactamase inhibitors were tested against cephalosporinase producing bacterial strains. The most significant antagonism was obtained with a combination of clavulanic acid and cefotaxime, while almost no antagonism was observed with sulbactam and tazobactam. In strains belonging to five different species there was a correlation between the levels of cephalosporinase produced after exposure to different concentrations of inhibitors and the MICs of cefotaxime combined with the same concentrations of inhibitors. It is concluded that there is little likelihood of antagonism between beta-lactam antibiotics and sulbactam or tazobactam.

In vitro antibacterial properties of cefetamet and in vivo activity of its orally absorbable ester derivative, cefetamet pivoxil

The in vitro activity of cefetamet, the microbiologically active metabolite of the orally administered prodrug cefetamet pivoxil, was compared with that of cephalexin, cefaclor, cefuroxime and amoxicillin. Cefetamet was highly active against Enterobacteriaceae, Neisseria spp., Vibrio spp., Haemophilus influenzae and streptococci other than enterococci. Cefetamet inhibited cefaclor-resistant species such as Proteus vulgaris, Providencia stuartii, Providencia rettgeri and Serratia marcescens. Staphylococci, Pseudomonas aeruginosa and cephalosporinase-overproducing strains of Enterobacter cloacae were resistant to cefetamet. The superior activity of cefetamet compared with older oral beta-lactam antibiotics against a large number of gram-negative pathogens correlated with enhanced stability towards beta-lactamases. In accordance with the in vitro findings, cefetamet pivoxil showed good activity in experimental infections in the mouse and rat, suggesting satisfactory bioavailability in these animals after oral administration.

Septicaemia caused by an Enterobacter cloacae strain varying in resistance against cephalosporins

Enterobacter cloacae, sensitive to third-generation cephalosporins (cefotaxime and ceftazidime), was isolated from the stoma of a patient with leukaemia. One month later, he developed a fatal septicaemia, caused by an identical strain isolated from blood cultures. He had been treated with several antibacterial agents, including cefotaxime. The blood culture strain seemed to be a mixture of four variants with different resistance patterns to cefotaxime and ceftazidime. One variant was extremely sensitive to third-generation cephalosporins, one was completely resistant, and two showed variations in zone diameter within sensitivity group 2, both for cefotaxime and ceftazidime. Minimal inhibitory concentration (MIC) studies also showed different resistance patterns between the four variants. Similar variants were found when the stoma isolate was further investigated.

Physiological studies of the regulation of beta-lactamase expression in Pseudomonas maltophilia

The kinetics of beta-lactamase induction in Pseudomonas maltophilia IID1275/873 were investigated. Upon induction with beta-lactam antibiotics, a correlation was seen between the increase in specific beta-lactamase activity and the generation time, as well as the concentration of inducer in the medium. The specific beta-lactamase activity increased slowly within the first 0.5 generation and then more rapidly; it decreased regularly after about 2 generations of growth in the presence of inducer. This decrease could presumably be attributed to the continuous breakdown of inducer by beta-lactamases in the culture medium. In a chemostat culture with continuous supply of fresh inducer-containing medium, the specific beta-lactamase activity could be stabilized at a high level over several generations. Removal of the beta-lactam after a certain induction time showed that a short exposure of the bacteria to inducer caused induction kinetics comparable to those resulting from continuous exposure of the cells to inducer. The two beta-lactamases of P. maltophilia, L1 and L2, were induced simultaneously under various experimental conditions.

Multiply beta-lactam resistant Enterobacter cloacae infections linked to the environmental flora in a unit for cardiothoracic and vascular surgery

During the period March 1987-May 1988, postoperative infection or colonization with Enterobacter cloacae occurred in 9/379 (2.4%) patients who underwent cardiovascular surgery. Five of the patients were infected with multiply beta-lactam resistant E. cloacae, of whom 4 had been infected with an identical, resistant strain during intervals of months. This strain was also found in the environmental flora of the cardiovascular operating suite and in a sink reservoir in the surgery department. All 4 patients with the identical resistant strain had serious complications during the postoperative period with symptoms of septicaemia in 3, multiorgan failure and shock in 2, and mediastinitis in 3. The single resistant strain of a different serotype was also associated with severe postoperative complications. The 4 sensitive strains were all different serotypes. None caused septicaemia, one was associated with mediastinitis, another with an uncomplicated sternum infection, and 2 were from sputum. In the 3 latter patients with sensitive strains and few postoperative complications, cephalosporins had not been used during the pre- or postoperative period.

In vitro interaction between the penem FCE 22101 and ceftazidime

FCE 22101, a penem antimicrobial agent, was found to resist hydrolysis by bacterial beta-lactamases and to have a strong affinity for Type Ia enzymes. Like imipenem, FCE 22101 was shown to be capable of inducing resistance to a wide range of beta-lactam antibiotics. FCE 22101 antagonized the in vitro activity of ceftazidime against enteric bacilli that commonly produce inducible enzymes. This penem should not be combined with other beta-lactams for chemotherapeutic purposes.

Clinical importance of inducible beta-lactamases in gram-negative bacteria

The clinical problems caused by inducible beta-lactamases in certain gram-negative bacteria are being recognized with increasing frequency. These problems include the rapid emergence of multiple beta-lactam resistance during therapy with many of the newer beta-lactam antibiotics. Such multiply resistant organisms are now spreading within the hospital and have become important nosocomial pathogens. This has been a particularly difficult problem for intensive care units, cystic fibrosis centers and burn units where there are clusters of patients who are highly susceptible to infections with organisms like Enterobacter spp., Serratia spp. and Pseudomonas aeruginosa, which possess inducible beta-lactamases. Only through an awareness of these problems, their cause, and restriction of the use of certain newer beta-lactam antibiotics can these problems be controlled.