A novel high-throughput screening method for identifying compounds that inhibit plasmid conjugation

Plasmid conjugation is an important contributing factor to the spread of antibiotic resistance among bacteria, posing a significant global health threat. Our method introduces an innovative high-throughput screening approach to identify compounds that inhibit or reduce conjugation, addressing the need for new strategies against the spread of antimicrobial resistance. Using Escherichia coli strains as donor and recipient, we screened 3500 compounds from a library provided by ABAC Therapeutics. Each 96 -well plate was loaded with 88 different compounds and bacterial cultures. Every plate also included negative and positive controls of conjugation. After an hour, cultures from wells were spotted on agar plates and assessed visually. Compounds that showed a visible effect on conjugation were retested. Six compounds targeting conjugation were found, showing promise for further analysis.

Enhancing the Efficacy of Chloramphenicol Therapy for Escherichia coli by Targeting the Secondary Resistome

The increasing prevalence of antimicrobial resistance and the limited availability of new antimicrobial agents have created an urgent need for new approaches to combat these issues. One such approach involves reevaluating the use of old antibiotics to ensure their appropriate usage and maximize their effectiveness, as older antibiotics could help alleviate the burden on newer agents. An example of such an antibiotic is chloramphenicol (CHL), which is rarely used due to its hematological toxicity. In the current study, we employed a previously published transposon mutant library in MG1655/pTF2::blaCTX-M-1, containing over 315,000 unique transposon insertions, to identify the genetic factors that play an important role during growth in the presence of CHL. The list of conditionally essential genes, collectively referred to as the secondary resistome (SR), included 67 genes. To validate our findings, we conducted gene knockout experiments on six genes: arcA, hfq, acrZ, cls, mdfA, and nlpI. Deleting these genes resulted in increased susceptibility to CHL as demonstrated by MIC estimations and growth experiments, suggesting that targeting the products encoded from these genes may reduce the dose of CHL needed for treatment and hence reduce the toxicity associated with CHL treatment. Thus, the gene products are indicated as targets for antibiotic adjuvants to favor the use of CHL in modern medicine.

Uncovering the Important Genetic Factors for Growth during Cefotaxime-Gentamicin Combination Treatment in blaCTX-M-1 Encoding Escherichia coli

Due to the rapid spread of CTX-M type ESBLs, the rate of resistance to third-generation cephalosporin has increased among Gram-negative bacteria, especially in Escherichia coli, and there is a need to find ways to re-sensitize ESBL E. coli to cephalosporin treatment. A previous study showed that genes involved in protein synthesis were significantly up-regulated in the presence of subinhibitory concentration of cefotaxime (CTX) in a CTX-M-1-producing E. coli. In this study, the interaction between CTX and gentamicin (GEN), targeting protein synthesis, was evaluated in MG1655/pTF2, and the MIC of CTX was strongly reduced (128-fold) in the presence of this combnation therapy. Since the underlying mechanism behind this synergy is not known, we constructed a saturated transposon mutant library in MG1655/pTF2::bla_CTX-M-1 containing 315,925 unique transposon insertions to measure mutant depletion upon exposure to CTX, GEN, and combination treatment of CTX and GEN by Transposon Directed Insertion-site Sequencing (TraDIS). We identified 57 genes that were depleted (log₂FC ≤ -2 and with q.value ≤ 0.01) during exposure to CTX, 18 for GEN, and 31 for combination treatment of CTX and GEN. For validation, we deleted eight genes that were either uniquely identified in combination treatment, overlapped with monotherapy of GEN, or were shared between combination treatment and monotherapy with CTX and GEN. Of these genes, we found that the inactivation of dnaK, mnmA, rsgA, and ybeD increased the efficacy of both CTX and GEN treatment, the inactivation of cpxR and yafN increased the efficacy of only CTX, and the inactivation of mnmA, rsgA, and ybeD resulted in increased synergy between CTX and GEN. Thus, the study points to putative targets for helper drugs that can restore susceptibility to these important drugs, and it indicates that genes involved in protein synthesis are essential for the synergy between these two drugs. In summary, the study identified mutants that sensitize ESBL-producing E. coli to CTX and a combination of CTX and GEN, and it increased our understanding of the mechanism behind synergy between β-lactam and aminoglycoside drugs. This forms a framework for developing new strategies to combat infections caused by resistant bacteria.

Characterizations of blaCTX-M-14 and blaCTX-M-64 in a clinical isolate of Escherichia coli from China

Extended-spectrum beta-lactamase-producing Gram-negative bacteria are common in the community and hospitals. To monitor ESBLs mediated by the CTX-M genotype, we collected clinical ESBL pathogenic strains from a hospital in central China and observed a strain of Escherichia coli, namely Ec15103 carrying blaCTX-M-14, blaCTX-M-64 and blaTEM-1, isolated from the blood of a 7-day-old infant in 2015. Strain Ec15103 contains two drug resistance plasmids: pEc15103A, an IncFI-type plasmid that cannot be conjugatively transferred and carries the drug resistance genes blaTEM-1, aacC2, aadA5, sul1, mph(A), sul2, strAB, and tetA(A); and pEc15103B, an IncK2/Z-type plasmid that carries the conjugation transfer gene and blaCTX-M-14. In addition, blaCTX-M-64 is located on the chromosome of Ec15103, and it is the first report of pathogen with blaCTX-M-64 located on its chromosome (the search terms used "blaCTX-M-64" and "chromosome"). blaCTX-M-14 and blaCTX-M-64 are carried by ISEcp1-mediated transposon Tn6503a and Tn6502, respectively. The conjugation transfer ability of pEc15103B was significantly inhibited by zidovudine (AZT) and linoleic acid (LA) and that expression of blaCTX-M-14, blaCTX-M-64 and blaTEM-1 at the mRNA level did not change based on the concentration of cefotaxime or ampicillin. Co-occurrence of blaCTX-M-14 and blaCTX-M-64 in a single isolate will enhance the drug resistance of bacteria, and the presence of blaCTX-M-64 in the chromosome may make the resistance more maintain. This fact will facilitate its dissemination and persistence under different antimicrobial selection pressures. It is essential to prevent these strains from further spreading in a hospital environment.

Comparative Study of ESBL Production Among Uropathogenic Escherichia coli Clinical Isolates from Pre- and Post-menopausal Women in Egypt

Urinary tract infection (UTI) is regarded one of the most frequent bacterial infections in women. Accordingly, the aim of the current study was to determine the prevalence of extended-spectrum beta-lactamase (ESBL), as well as the degree of antimicrobial resistance among premenopausal (n = 44) and postmenopausal (n = 49) women suffering from uncomplicated UTI. Urinary samples (n = 93) collected from women with UTI were tested for their antimicrobial sensitivity and assessed for ESBL production by both phenotypic and genotypic methods. Phenotypically, the presence of ESBL was observed in 64 isolates, while polymerase chain reaction detected ESBL-encoding genes in 57 isolates. The CTX-M gene was the most predominant (51.6%), followed by TEM (46.2%), and the SHV gene (17.2%). Surprisingly, all ESBL-producing Escherichia coli isolates were multidrug-resistant (MDR). To the best of our knowledge, this is the first study conducted in Egypt showing significant correlation between ESBL production, multidrug resistance and menopausal state in women. The results demonstrate alarming signal for the dissemination of ESBL genes among uropathogenic E. coli that are MDR in Egypt.

Selection of ESBL-producing Escherichia coli in the gut of calves experimentally fed with milk containing antibiotic residues

In the bovine sector, the spread of Enterobacterales producing extended-spectrum and AmpC β-lactamases (ESBL/AmpC) mostly concerns veal calves, and the use of waste milk containing antibiotic residues has been recurrently incriminated. In this study, calves were experimentally fed with milk containing either 2,000 μg/L or 20,000 μg/L of the critically important antibiotic cefquinome. The total counts of enterobacterales and ESBL-producing E. coli were monitored using non-selective and selective media. Our data highlighted the important combination of two main factors (cefquinome exposure and initial ESBL colonization level) in the ESBL selection and amplification process in the gut of calves. Results also proved the dose-independent effect of cefquinome administration on the selection and amplification of ESBL-producing E. coli. Finally, the bla_CTX-M-1/IncI1 ST3 plasmid was systematically recovered after cefquinome exposure, highlighting its epidemic success. Altogether, this work is one of the rare experimental studies providing quantitative information on the impact of waste milk containing antimicrobials on the ESBL load in calves' microbiota, and the first one using cefquinome. These data emphasise the need for global guidelines on the use of waste milk on dairy farms in order to decrease the antimicrobial resistance burden in this sector.

Molecular detection of beta-lactamase blaCTX-M group 1 in Escherichia coli isolated from drinking water in Khartoum State

This study aimed to detect the bla_CTX-M group 1 in Escherichia coli (E. coli) isolated from drinking water in Khartoum State. Two hundred and eighty water samples were collected randomly from different areas, places, and sources from the state and examined for the presence of E. coli as a fecal contamination indicator. Isolation and identification of E. coli were performed using culture characteristics on different culture media and biochemical reactions. An antimicrobial sensitivity test was performed for all isolated E. coli using agar disk diffusion method. DNA was extracted by boiling method, and bacterial genomic DNA used as a template to detect bla_CTX-M group 1 by PCR. Results showed 86 (30.7%) E. coli were isolated out of 280 water samples. Antimicrobial susceptibility testing revealed the highest resistant percentage was 59% for tetracycline, followed by 35% for gentamycin, while for chloramphenicol and cefotaxime was 22 and 20%, respectively. bla_CTX-M group 1 was detected in about 40% of all isolates. This study concludes that drinking water in Khartoum State may be contaminated with feces and might be a possible source for transferring resistant bacteria. Thus, it may be one of the critical causes of increasing reports of antimicrobial resistance in Khartoum State.

Profile of Enterobacteria Resistant to Beta-Lactams

A serious emerging problem worldwide is increased antimicrobial resistance. Acquisition of coding genes for evasion methods of antimicrobial drug mechanisms characterizes acquired resistance. This phenomenon has been observed in Enterobacteriaceae family. Treatment for bacterial infections is performed with antibiotics, of which the most used are beta-lactams. The aim of this study was to correlate antimicrobial resistance profiles in Enterobacteriaceae by phenotypic methods and molecular identification of 14 beta-lactamase coding genes. In this study, 70 exclusive isolates from Brazil were used, half of which were collected in veterinary clinics or hospitals Phenotypic methodologies were used and real-time PCR was the molecular methodology used, through the Sybr Green system. Regargding the results found in the tests it was observed that 74.28% were resistant to ampicillin, 62.85% were resistant to amoxicillin associated with clavalunate. The mechanism of resistance that presented the highest expression was ESBL (17.14%). The genes studied that were detected in a greater number of species were blaGIM and blaSIM (66.66% of the samples) and the one that was amplified in a smaller number of samples was blaVIM (16.66%). Therefore, high and worrying levels of antimicrobial resistance have been found in enterobacteria, and a way to minimize the accelerated emergence of their resistance includes developing or improving techniques that generate diagnoses with high efficiency and speed.

Influence of zinc on CTX-M-1 β-lactamase expression in Escherichia coli

OBJECTIVE

Zinc oxide is used to prevent post-weaning diarrhoea in pigs as an alternative to antimicrobial growth promoters. This study aims to determine if the use of zinc oxide selects for extended-spectrum β-lactamase (ESBL)-producing Escherichia coli and affects the expression of bla_CTX-M-1 in E. coli.

METHODS

Using an in vitro faecal micro-cosmos model, the selective properties of zinc were investigated using an E. coli strain with bla_CTX-M-1 encoded by a natural IncI1 resistance plasmid (MG1655/pTF2) and another strain where the same gene was located on the chromosome (MG1655::bla_CTX-M-1). The micro-cosmos was seeded with faecal material containing an increasing concentration of zinc (0-8 mM). Outcome measurements consisted of colony-forming units (CFU) of the inoculated ESBL E. coli and naturally occurring coliforms as determined by plate counting on MacConkey with and without 5 mg/L cefotaxime as well as total viable bacteria determined on Luria agar without cefotaxime. Expression of bla_CTX-M-1 under the experimental zinc concentrations was determined by quantitative polymerase chain reaction.

RESULTS

The proportion of MG1655/pTF2 of the total viable bacteria was significantly higher at high zinc concentrations (6 and 8 mM) compared with low concentrations (0-4 mM). The messenger RNA (mRNA) levels of bla_CTX-M-1 in the two ESBL strains increased at increasing zinc concentrations and varied with the growth phase.

CONCLUSION

The growth of the inoculated CTX-M-1-encoding E. coli MG1655 strains and naturally occurring coliforms was impacted differently when exposed to zinc oxide. The bla_CTX-M-1 mRNA expression levels seemed to increase with increasing zinc concentrations, but varied with growth phase, but not gene location.

Extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae in cattle production - a threat around the world

Food producing animal is a global challenge in terms of antimicrobial resistance spread. Extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae are relevant opportunistic pathogens that may spread in many ecological niches of the One Health approach as human, animal and environment due to intestinal selection of antimicrobial resistant commensals in food production animals. Cattle production is a relevant ecological niche for selection of commensal bacteria with antimicrobial resistance from microbiota. Enterobacteriaceae show importance in terms of circulation of resistant-bacteria and antimicrobial resistance genes via food chain creating a resistance reservoir, setting up a threat for colonization of humans and consequent health risk. ESBL-producing Enterobacteriaceae are a threat in terms of human health responsible for life threatening outbreaks and silent enteric colonization of community populations namely the elder population. Food associated colonization is a risk difficult to handle and control. In a time of globalization of food trading, population intestinal colonization is a mirror of food production and in that sense this work aims to make a picture of ESBL-producing Enterobacteriaceae in animal production for food over the world in order to make some light in this reality of selection of resistant threats in food producing animal.

Identification of Genes Essential for Antibiotic-Induced Up-Regulation of Plasmid-Transfer-Genes in Cephalosporin Resistant Escherichia coli

Bacterial conjugation is one of the most important mechanisms for spread of antibiotic resistance among bacteria. We have previously demonstrated that cefotaxime (CTX) exposure up-regulates expression of Type-IV conjugation transfer genes, and that this leads to increased transfer of a bla _{CTX-M- 1} encoding IncI1 resistance plasmid pTF2 in Escherichia coli. To elucidate the underlying mechanisms, a search for genes that are essential for the up-regulated expression of the transfer (tra) genes in the presence of CTX was undertaken. We constructed a reporter gene-fusion strain MG1655/pTF2 ΔtraF:lacZ where the promoter region of the traF-gene of the plasmid pTF2 was fused with a lacZ on the native plasmid. Random mutagenesis mediated by Tn5 transposon was carried out in the strain, and seven genes (rfaH, yhiN, waaP, waaQ, gnd, pgl, and ISEcp1) were identified where insertion prevented CTX-induced up regulation of traF. Site-specific mutagenesis was carried out, and for all seven mutants, gene deletions abolished the CTX induced up-regulation of traF, and the increased conjugation transfer of the plasmid in the presence of CTX was no longer observed. In addition, the deletion of the genes also abolished CTX induced expression of the bla _{CTX-M- 1} gene. Our results suggested that through CTX induced induction of the identified genes, bla _{CTX-M- 1} expression increased, which led to up-regulation of traF and plasmid transfer. These data reveal that a number of chromosomally encoded genes contribute to the antibiotic induced up-regulation of the conjugation machinery of plasmids, and such genes may be future targets to prevent antibiotic induced spread of resistance plasmids.

Antibiotic-Induced, Increased Conjugative Transfer Is Common to Diverse Naturally Occurring ESBL Plasmids in Escherichia coli

Previously, we showed that cefotaxime (CTX) exposure increases conjugative transfer of a bla CTX-M- 1 encoding IncI1 plasmid (IncI1/pST49/CTX-M-1) in Escherichia coli in a SOS-independent manner. This study aimed at investigating whether the observation was unique for that plasmid/strain/antibiotic combination or whether antibiotic-induced plasmid transfer (PT) is a more general phenomenon among plasmids in E. coli. Whole genome sequences of 25 E. coli strains were analyzed to identify different extended spectrum beta-lactamases (ESBL) plasmids enabling selection of a diverse collection of plasmids. Experiments were performed following exposure of these strains to 1/2 minimal inhibitory concentration (MIC) of CTX, ampicillin (AMP), or ciprofloxacin (CIP) before conjugation experiments. The frequency of PT was measured and compared to that of donors not exposed to antibiotics. Reverse-transcribed-quantitative real time polymerase chain reaction (RT-qPCR) was used to measure mRNA levels of five PT genes and two SOS response genes in donors exposed to antibiotics. The PT of eight strains (30.8% of strains tested) with IncI1/pST7/CTX-M-1, IncI1/pST49/CTX-M-1, IncI1/pST3/CTX-M-1, IncI1/pST293/CTX-M-1, IncI1/pST295/CTX-M-1, IncI1/pST16/CTX-M-55, and IncFII/CTX-M-14 (n = 2) plasmids was significantly increased following antibiotic exposure. CTX increased PT in all of these eight strain/plasmid combinations, AMP and CIP increased the PT in six and three strains, respectively. RT-qPCR showed that PT genes were up-regulated in the presence of the three antibiotics, whereas SOS-response genes were up-regulated only following CIP exposure. Our findings reveal that antibiotics can increase PT in E. coli strains with various ESBL plasmids. Thus, antibiotic-induced conjugative transfer of ESBL plasmids appears to be a common phenomenon in E. coli, having important implications for assessing the risks of antibiotic use.

Applying ecological resistance and resilience to dissect bacterial antibiotic responses

An essential property of microbial communities is the ability to survive a disturbance. Survival can be achieved through resistance, the ability to absorb effects of a disturbance without a notable change, or resilience, the ability to recover after being perturbed by a disturbance. These concepts have long been applied to the analysis of ecological systems, although their interpretations are often subject to debate. Here, we show that this framework readily lends itself to the dissection of the bacterial response to antibiotic treatment, where both terms can be unambiguously defined. The ability to tolerate the antibiotic treatment in the short term corresponds to resistance, which primarily depends on traits associated with individual cells. In contrast, the ability to recover after being perturbed by an antibiotic corresponds to resilience, which primarily depends on traits associated with the population. This framework effectively reveals the phenotypic signatures of bacterial pathogens expressing extended-spectrum β-lactamases (ESBLs) when treated by a β-lactam antibiotic. Our analysis has implications for optimizing treatment of these pathogens using a combination of a β-lactam and a β-lactamase (Bla) inhibitor. In particular, our results underscore the need to dynamically optimize combination treatments based on the quantitative features of the bacterial response to the antibiotic or the Bla inhibitor.

Physicochemical Factors That Favor Conjugation of an Antibiotic Resistant Plasmid in Non-growing Bacterial Cultures in the Absence and Presence of Antibiotics

Horizontal gene transfer (HGT) of antibiotic resistance genes has received increased scrutiny from the scientific community in recent years owing to the public health threat associated with antibiotic resistant bacteria. Most studies have examined HGT in growing cultures. We examined conjugation in growing and non-growing cultures of E. coli using a conjugative multi antibiotic and metal resistant plasmid to determine physiochemical parameters that favor horizontal gene transfer. The conjugation frequency in growing and non-growing cultures was generally greater under shaken than non-shaken conditions, presumably due to increased frequency of cell collisions. Non-growing cultures in 9.1 mM NaCl had a similar conjugation frequency to that of growing cultures in Luria-Bertaini broth, whereas those in 1 mM or 90.1 mM NaCl were much lower. This salinity effect on conjugation was attributed to differences in cell-cell interactions and conformational changes in cell surface macromolecules. In the presence of antibiotics, the conjugation frequencies of growing cultures did not increase, but in non-growing cultures of 9.1 mM NaCl supplemented with Cefotaxime the conjugation frequency was as much as nine times greater than that of growing cultures. The mechanism responsible for the increased conjugation in non-growing bacteria was attributed to the likely lack of penicillin-binding protein 3 (the target of Cefotaxime), in non-growing cells that enabled Cefotaxime to interact with the plasmid and induce conjugation. Our results suggests that more attention may be owed to HGT in non-growing bacteria as most bacteria in the environment are likely not growing and the proposed mechanism for increased conjugation may not be unique to the bacteria/plasmid system we studied.

Antimicrobial Resistance and Molecular Characterization of Extended-Spectrum β-Lactamases of Escherichia coli and Klebsiella spp. Isolates from Urinary Tract Infections in Southern Brazil

The objective of this study was to evaluate the frequency of different extended-spectrum β-lactamases (ESBL) as well as to associate these ESBL with antimicrobial (ATM) resistance in Escherichia coli and Klebsiella spp. isolates from outpatients and inpatients with urinary tract infections. The study included 435 consecutive nonduplicate clinical isolates, including 362 E. coli isolates, 62 Klebsiella pneumoniae isolates, and 11 K. oxytoca isolates. Isolates were obtained from patients who were treated in a University Hospital between August 2012 and July 2013. Three multiplex PCR were performed to identify the ESBL groups. A total of 48 (11%) ESBL-producing isolates were found. The risk for the ESBL presence was significantly higher in males (26.4%) than females (8%), from hospital-acquired infections (29.1%) than community-acquired infections (7.0%) and in Klebsiella spp. (27.4%) than in E. coli (7.7%). ESBL-producing isolates presented a significantly higher percentage of resistance in 21 of the 23 ATMs analyzed. The CTX-M-1 group was the most predominant ESBL identified. The bla_{CTX-M-1-group} gene was found in 56% of the total ESBL producers from community and in 42.4% from hospital origins; it was followed in frequency by the bla_{CTX-M-8/25-group}, also found in both environments. Klebsiella spp. presented the largest variety of β-lactamase enzyme combinations and a higher level of resistance to cefotaxime. These findings contribute to better knowledge of the epidemiology of ESBL enzymes and are alarming for the reduced therapeutic options available for the risk groups identified in the studied populations.

A culture-independent method for studying transfer of IncI1 plasmids from wild-type Escherichia coli in complex microbial communities

IncI1 plasmids play a central role in the transfer of antimicrobial resistance genes among Enterobacteriaceae in animals and humans. Knowledge on the dynamics of IncI1 plasmid transfer is limited, mainly due to lack of culture-independent methods that can quantify donor strain survival and plasmid transfer in complex microbial communities. The aim of this study was to develop a culture-independent method to study the dynamics of IncI1 plasmids transfer by fluorescence-activated cell sorting. We genetically modified three wild-type Escherichia coli of animal (n = 2) and human (n = 1) origin carrying bla_CMY-2 or bla_CTX-M-1 on two epidemic IncI1 plasmids (pST12 and pST7). Non-coding regions on the chromosome and on the IncI1 plasmid of each strain were tagged with mCherry (red) and GFPmut3 (green) fluorescent proteins, respectively, using lambda recombineering. A gene cassette expressing mCherry and lacI^q was inserted into the chromosome, whereas the plasmid was marked with a GFPmut3 cassette with LacI^q repressible promoter. Therefore, gfpmut3 was repressed in donor strains but expressed in recipient strains acquiring the plasmids. We demonstrated that genetic engineering of the strains did not affect the growth rate and plasmid transfer-ability in filter and broth matings. A proof-of-concept experiment using the CoMiniGut, an in vitro model of the colon, proved the validity of our method for studying the survival of wild-type E. coli and horizontal transfer of IncI1 plasmids under different pH and oxygen conditions. The dual-labeling method by fluorescent proteins is useful to determine persistence of exogenous E. coli and transfer dynamics of IncI1 plasmids in microbial communities.

Treatment with Cefotaxime Affects Expression of Conjugation Associated Proteins and Conjugation Transfer Frequency of an IncI1 Plasmid in Escherichia coli

Horizontal gene transfer (HGT) is the major mechanism responsible for spread of antibiotic resistance. Antibiotic treatment has been suggested to promote HGT, either by directly affecting the conjugation process itself or by selecting for conjugations subsequent to DNA transfer. However, recent research suggests that the effect of antibiotic treatment on plasmid conjugation frequencies, and hence the spread of resistance plasmids, may have been overestimated. We addressed the question by quantifying transfer proteins and conjugation frequencies of a blaCTX-M-1 encoding IncI1 resistance plasmid in Escherichia coli MG1655 in the presence and absence of therapeutically relevant concentrations of cefotaxime (CTX). Analysis of the proteome by iTRAQ labeling and liquid chromatography tandem mass spectrometry revealed that Tra proteins were significantly up-regulated in the presence of CTX. The up-regulation of the transfer machinery was confirmed at the transcriptional level for five selected genes. The CTX treatment did not cause induction of the SOS-response as revealed by absence of significantly regulated SOS associated proteins in the proteome and no significant up-regulation of recA and sfiA genes. The frequency of plasmid conjugation, measured in an antibiotic free environment, increased significantly when the donor was pre-grown in broth containing CTX compared to growth without this drug, regardless of whether blaCTX-M-1 was located on the plasmid or in trans on the chromosome. The results shows that antibiotic treatment can affect expression of a plasmid conjugation machinery and subsequent DNA transfer.

Relation between tetR and tetA expression in tetracycline resistant Escherichia coli

BACKGROUND

Tetracyclines are among the most used antibiotics in livestock worldwide. Resistance is widely disseminated in Escherichia coli, where it is generally mediated by tetracycline efflux pumps, such as TetA. Expression of tetracycline efflux pumps is tightly controlled by the repressor TetR, which has been shown to be tetracycline-responsive at sub-MIC tetracycline concentrations. The objective of this study was to investigate the effects of increasing tetracycline concentrations on the growth of TetA-producing E. coli, and to determine how expression of tetA and tetR related to each other in different growth phases in the presence of tetracycline.

RESULTS

A tetracycline resistant E. coli strain containing tetA and tetR on the chromosome was constructed and cultured in the presence of increasing concentrations of tetracycline. Expression of tetR and tetA was measured at four time points in different growth phases by quantitative real-time PCR. The TetA-producing E. coli exhibited prolonged lag phase with increasing concentrations of tetracycline, while expression of tetA and tetR increased and decreased, respectively, with increasing tetracycline concentration. The levels of tetA and tetR mRNA varied depending on growth phase, resulting in a gradual decrease of the tetA/tetR ratio from approximately 4 in the lag phase to approximately 2 in the stationary phase.

CONCLUSION

This study shows that the expression of tetR and tetA is tetracycline concentration- and growth phase-dependent, contributing to improved understanding of the relationships between E. coli growth, tetracycline exposure and expression of tetracycline resistance.

Extended spectrum β-lactamase-producing Escherichia coli forms filaments as an initial response to cefotaxime treatment

Background

β-lactams target the peptidoglycan layer in the bacterial cell wall and most β-lactam antibiotics cause filamentation in susceptible Gram-negative bacteria at low concentrations. The objective was to determine the initial morphological response of cephalosporin resistant CTX-M-1-producing E. coli to cefotaxime and to determine whether the response depended on the growth phase of the bacterium and the concentration of antibiotic.

Results

Two antibiotic resistant strains carrying bla_CTX-M-1 on the chromosome and on an IncI1 plasmid and three sensitive strains were used in this study. The resistant strains displayed elongated cells when exposed to cefotaxime at sub-inhibitory as well as therapeutic concentrations (1 to 512 mg/L of cefotaxime) in both lag and early exponential phase, suggesting that the elongation was an initial response mechanism to the antibiotic. Normal sized cells were the dominant cell type in exponential and stationary growth phase. No elongated cells were seen in cultures without cefotaxime. In cultures with high concentrations of cefotaxime (128–512 mg/L), no growth other than initial filamentation was observed, but spheroplats appeared after 14–17 hours in cultures of the resistant strains. Filaments were also observed in sensitive control strains with sub-inhibitory concentrations of cefotaxime.

Conclusions

We showed that E. coli resistant to β-lactams by an extended-spectrum β-lactamase, bla_CTX-M-1, produced filaments when exposed to cefotaxime. The filament formation was restricted to early growth phases and the time the cells grew as filaments was antibiotic concentration dependent. This indicates that antibiotic resistant E. coli undergo the same morphological changes as sensitive bacteria in the presence of β-lactam antibiotic. It was showed that the filament formation was an initial response to the antibiotics.