Synthesis of metallo-β-lactamase VIM-2 is associated with a fitness reduction in Salmonella enterica Serovar Typhimurium

Filling knowledge gaps related to AmpC-dependent β-lactam resistance in Enterobacter cloacae

Enterobacter cloacae starred different pioneer studies that enabled the development of a widely accepted model for the peptidoglycan metabolism-linked regulation of intrinsic class C cephalosporinases, highly conserved in different Gram-negatives. However, some mechanistic and fitness/virulence-related aspects of E. cloacae choromosomal AmpC-dependent resistance are not completely understood. The present study including knockout mutants, β-lactamase cloning, gene expression analysis, characterization of resistance phenotypes, and the Galleria mellonella infection model fills these gaps demonstrating that: (i) AmpC enzyme does not show any collateral activity impacting fitness/virulence; (ii) AmpC hyperproduction mediated by ampD inactivation does not entail any biological cost; (iii) alteration of peptidoglycan recycling alone or combined with AmpC hyperproduction causes no attenuation of E. cloacae virulence in contrast to other species; (iv) derepression of E. cloacae AmpC does not follow a stepwise dynamics linked to the sequential inactivation of AmpD amidase homologues as happens in Pseudomonas aeruginosa; (v) the enigmatic additional putative AmpC-type β-lactamase generally present in E. cloacae does not contribute to the classical cephalosporinase hyperproduction-based resistance, having a negligible impact on phenotypes even when hyperproduced from multicopy vector. This study reveals interesting particularities in the chromosomal AmpC-related behavior of E. cloacae that complete the knowledge on this top resistance mechanism.

The balance between antibiotic resistance and fitness/virulence in Pseudomonas aeruginosa: an update on basic knowledge and fundamental research

The interplay between antibiotic resistance and bacterial fitness/virulence has attracted the interest of researchers for decades because of its therapeutic implications, since it is classically assumed that resistance usually entails certain biological costs. Reviews on this topic revise the published data from a general point of view, including studies based on clinical strains or in vitro-evolved mutants in which the resistance phenotype is seen as a final outcome, i.e., a combination of mechanisms. However, a review analyzing the resistance/fitness balance from the basic research perspective, compiling studies in which the different resistance pathways and respective biological costs are individually approached, was missing. Here we cover this gap, specifically focusing on Pseudomonas aeruginosa, a pathogen that stands out because of its extraordinary capacity for resistance development and for which a considerable number of recent and particular data on the interplay with fitness/virulence have been released. The revised information, split into horizontally-acquired vs. mutation-driven resistance, suggests a great complexity and even controversy in the resistance-fitness/virulence balance in the acute infection context, with results ranging from high costs linked to certain pathways to others that are seemingly cost-free or even cases of resistance mechanisms contributing to increased pathogenic capacities. The elusive mechanistic basis for some enigmatic data, knowledge gaps, and possibilities for therapeutic exploitation are discussed. The information gathered suggests that resistance-fitness/virulence interplay may be a source of potential antipseudomonal targets and thus, this review poses the elementary first step for the future development of these strategies harnessing certain resistance-associated biological burdens.

Role of Enzymatic Activity in the Biological Cost Associated with the Production of AmpC β-Lactamases in Pseudomonas aeruginosa

In the current scenario of growing antibiotic resistance, understanding the interplay between resistance mechanisms and biological costs is crucial for designing therapeutic strategies. In this regard, intrinsic AmpC β-lactamase hyperproduction is probably the most important resistance mechanism of Pseudomonas aeruginosa, proven to entail important biological burdens that attenuate virulence mostly under peptidoglycan recycling alterations. P. aeruginosa can acquire resistance to new β-lactam-β-lactamase inhibitor combinations (ceftazidime-avibactam and ceftolozane-tazobactam) through mutations affecting ampC and its regulatory genes, but the impact of these mutations on the associated biological cost and the role that β-lactamase activity plays per se in contributing to the above-mentioned virulence attenuation are unknown. The same questions remain unsolved for plasmid-encoded AmpC-type β-lactamases such as FOX enzymes, some of which also provide resistance to new β-lactam-β-lactamase inhibitor combinations. Here, we assessed from different perspectives the effects of changes in the active center and, thus, in the hydrolytic spectrum resistance to inhibitors of AmpC-type β-lactamases on the fitness and virulence of P. aeruginosa, using site-directed mutagenesis; the previously described AmpC variants T96I, G183D, and ΔG229-E247; and, finally, blaFOX-4 versus blaFOX-8. Our results indicate the essential role of AmpC activity per se in causing the reported full virulence attenuation (in terms of growth, motility, cytotoxicity, and Galleria mellonella larvae killing), although the biological cost of the above-mentioned AmpC-type variants was similar to that of the wild-type enzymes. This suggests that there is not an important biological burden that may limit the selection/spread of these variants, which could progressively compromise the future effectiveness of the above-mentioned drug combinations. IMPORTANCE The growing antibiotic resistance of the top nosocomial pathogen Pseudomonas aeruginosa pushes research to explore new therapeutic strategies, for which the resistance-versus-virulence balance is a promising source of targets. While resistance often entails significant biological costs, little is known about the bases of the virulence attenuations associated with a resistance mechanism as extraordinarily relevant as β-lactamase production. We demonstrate that besides potential energy and cell wall alterations, the enzymatic activity of the P. aeruginosa cephalosporinase AmpC is essential for causing the full attenuation associated with its hyperproduction by affecting different features related to pathogenesis, a fact exploitable from the antivirulence perspective. Less encouraging, we also show that the production of different chromosomal/plasmid-encoded AmpC derivatives conferring resistance to some of the newest antibiotic combinations causes no significantly increased biological burdens, which suggests a free way for the selection/spread of these types of variants, potentially compromising the future effectiveness of these antipseudomonal therapies.

Impact of Peptidoglycan Recycling Blockade and Expression of Horizontally Acquired β-Lactamases on Pseudomonas aeruginosa Virulence

In the current scenario of antibiotic resistance magnification, new weapons against top nosocomial pathogens like Pseudomonas aeruginosa are urgently needed. The interplay between β-lactam resistance and virulence is considered a promising source of targets to be attacked by antivirulence therapies, and in this regard, we previously showed that a peptidoglycan recycling blockade dramatically attenuated the pathogenic power of P. aeruginosa strains hyperproducing the chromosomal β-lactamase AmpC. Here, we sought to ascertain whether this observation could be applicable to other β-lactamases. To do so, P. aeruginosa wild-type or peptidoglycan recycling-defective strains (ΔampG and ΔnagZ) harboring different cloned β-lactamases (transferable GES, VIM, and OXA types) were used to assess their virulence in Galleria mellonella larvae by determining 50% lethal doses (LD50s). A mild yet significant LD50 increase was observed after peptidoglycan recycling disruption per se, whereas the expression of class A and B enzymes did not impact virulence. While the production of the narrow-spectrum class D OXA-2 entailed a slight attenuation, its extended-spectrum derivatives OXA-226 (W159R [bearing a change of W to R at position 159]), OXA-161 (N148D), and principally, OXA-539 (D149 duplication) were associated with outstanding virulence impairments, especially in recycling-defective backgrounds (with some LD50s being >1,000-fold that of the wild type). Although their exact molecular bases remain to be deciphered, these results suggest that mutations affecting the catalytic center and, therefore, the hydrolytic spectrum of OXA-2-derived enzymes also drastically impact the pathogenic power of P. aeruginosa. This work provides new and relevant knowledge to the complex topic of the interplay between the production of β-lactamases and virulence that could be useful to build future therapeutic strategies against P. aeruginosa. IMPORTANCE Pseudomonas aeruginosa is one of the leading nosocomial pathogens whose growing resistance makes the development of therapeutic options extremely urgent. The resistance-virulence interplay has classically aroused researchers' interest as a source of therapeutic targets. In this regard, we describe a wide array of virulence attenuations associated with different transferable β-lactamases, among which the production of OXA-2-derived extended-spectrum β-lactamases stood out as a dramatic handicap for pathogenesis, likely as a side effect of mutations causing the expansion of their hydrolytic spectrums. Moreover, our results confirm the validity of disturbing peptidoglycan recycling as a weapon to attenuate P. aeruginosa virulence in class C and D β-lactamase production backgrounds. In the current scenario of dissemination of horizontally acquired β-lactamases, this work brings out new data on the complex interplay between the production of specific enzymes and virulence attenuation that, if complemented with the characterization of the underlying mechanisms, will likely be exploitable to develop future virulence-targeting antipseudomonal strategies.

OXA-23 β-Lactamase Overexpression in Acinetobacter baumannii Drives Physiological Changes Resulting in New Genetic Vulnerabilities

β-Lactamase expression is the major mechanism of resistance to penicillins, cephalosporins, and carbapenems in the multidrug-resistant (MDR) bacterium Acinetobacter baumannii. In fact, stable high-level expression of at least one β-lactamase has been rapidly increasing and reported to occur in up to 98.5% of modern A. baumannii isolates recovered in the clinic. Moreover, the OXA-51 β-lactamase is universally present in the A. baumannii chromosome, suggesting it may have a cellular function beyond antibiotic resistance. However, the consequences associated with OXA β-lactamase overexpression on A. baumannii physiology are not well understood. Using peptidoglycan composition analysis, we show that overexpressing the OXA-23 β-lactamase in A. baumannii drives significant collateral changes with alterations consistent with increased amidase activity. Consequently, we predicted that these changes create new cellular vulnerabilities. As proof of principle, a small screen of random transposon insertions revealed three genes, where mutations resulted in a greater than 19-fold loss of viability when OXA-23 was overexpressed. The identified genes remained conditionally essential even when a catalytically inactive OXA-23 β-lactamase was overexpressed. In addition, we demonstrated a synergistic lethal relationship between OXA-23 overexpression and a CRISPR interference (CRISPRi) knockdown of the essential peptidoglycan synthesis enzyme MurA. Last, OXA-23 overexpression sensitized cells to two inhibitors of peptidoglycan synthesis, d-cycloserine and fosfomycin. Our results highlight the impact of OXA-23 hyperexpression on peptidoglycan integrity and reveal new genetic vulnerabilities, which may represent novel targets for antimicrobial agents specific to MDR A. baumannii and other OXA β-lactamase-overexpressing Enterobacteriaceae, while having no impact on the normal flora. IMPORTANCE Acinetobacter baumannii has become a serious pathogen in both hospital and community settings. The β-lactam class of antibiotics is a primary treatment option for A. baumannii infections, and expression of β-lactamases is the most frequent mechanism of resistance in this bacterium. New approaches to treating multidrug-resistant A. baumannii strains are needed. In this study, we demonstrate that overexpressing the OXA-23 β-lactamase leads to significant collateral changes, where peptidoglycan structure is altered. We have identified genes that become selectively essential in OXA-23-expressing strains and confirmed the relationship between altered peptidoglycan and OXA-23 expression by demonstrating that OXA-23 overexpression sensitizes cells to genetic and chemical inhibition of peptidoglycan synthesis. This work paves the way for the identification of new antimicrobial targets, where inhibitors would selectively kill β-lactamase-expressing strains.

Metallo-β-lactamases in the Age of Multidrug Resistance: From Structure and Mechanism to Evolution, Dissemination, and Inhibitor Design

Antimicrobial resistance is one of the major problems in current practical medicine. The spread of genes coding for resistance determinants among bacteria challenges the use of approved antibiotics, narrowing the options for treatment. Resistance to carbapenems, last resort antibiotics, is a major concern. Metallo-β-lactamases (MBLs) hydrolyze carbapenems, penicillins, and cephalosporins, becoming central to this problem. These enzymes diverge with respect to serine-β-lactamases by exhibiting a different fold, active site, and catalytic features. Elucidating their catalytic mechanism has been a big challenge in the field that has limited the development of useful inhibitors. This review covers exhaustively the details of the active-site chemistries, the diversity of MBL alleles, the catalytic mechanism against different substrates, and how this information has helped developing inhibitors. We also discuss here different aspects critical to understand the success of MBLs in conferring resistance: the molecular determinants of their dissemination, their cell physiology, from the biogenesis to the processing involved in the transit to the periplasm, and the uptake of the Zn(II) ions upon metal starvation conditions, such as those encountered during an infection. In this regard, the chemical, biochemical and microbiological aspects provide an integrative view of the current knowledge of MBLs.

Protein determinants of dissemination and host specificity of metallo-β-lactamases

The worldwide dissemination of metallo-β-lactamases (MBLs), mediating resistance to carbapenem antibiotics, is a major public health problem. The extent of dissemination of MBLs such as VIM-2, SPM-1 and NDM among Gram-negative pathogens cannot be explained solely based on the associated mobile genetic elements or the resistance phenotype. Here, we report that MBL host range is determined by the impact of MBL expression on bacterial fitness. The signal peptide sequence of MBLs dictates their adaptability to each host. In uncommon hosts, inefficient processing of MBLs leads to accumulation of toxic intermediates that compromises bacterial growth. This fitness cost explains the exclusion of VIM-2 and SPM-1 from Escherichia coli and Acinetobacter baumannii, and their confinement to Pseudomonas aeruginosa. By contrast, NDMs are expressed without any apparent fitness cost in different bacteria, and are secreted into outer membrane vesicles. We propose that the successful dissemination and adaptation of MBLs to different bacterial hosts depend on protein determinants that enable host adaptability and carbapenem resistance.

Metallo-β-lactamases (MBLs) confer resistance to carbapenem antibiotics. Here, López et al. show that the host range of MBLs depends on the efficiency of MBL signal peptide processing and secretion into outer membrane vesicles, which affects bacterial fitness.

Detection of VIM-1-Producing Enterobacter cloacae and Salmonella enterica Serovars Infantis and Goldcoast at a Breeding Pig Farm in Germany in 2017 and Their Molecular Relationship to Former VIM-1-Producing S. Infantis Isolates in German Livestock Production

In 2011, VIM-1-producing Salmonella enterica serovar Infantis and Escherichia coli were isolated for the first time in four German livestock farms. In 2015/2016, highly related isolates were identified in German pig production. This raised the issue of potential reservoirs for these isolates, the relation of their mobile genetic elements, and potential links between the different affected farms/facilities. In a piglet-producing farm suspicious for being linked to some blaVIM-1 findings in Germany, fecal and environmental samples were examined for the presence of carbapenemase-producing Enterobacteriaceae and Salmonella spp. Newly discovered isolates were subjected to Illumina whole-genome sequencing (WGS) and S1 pulsed-field gel electrophoresis (PFGE) hybridization experiments. WGS data of these isolates were compared with those for the previously isolated VIM-1-producing Salmonella Infantis isolates from pigs and poultry. Among 103 samples, one Salmonella Goldcoast isolate, one Salmonella Infantis isolate, and one Enterobacter cloacae isolate carrying the blaVIM-1 gene were detected. Comparative WGS analysis revealed that the blaVIM-1 gene was part of a particular Tn21-like transposable element in all isolates. It was located on IncHI2 (ST1) plasmids of ∼290 to 300 kb with a backbone highly similar (98 to 100%) to that of reference pSE15-SA01028. SNP analysis revealed a close relationship of all VIM-1-positive S Infantis isolates described since 2011. The findings of this study demonstrate that the occurrence of the blaVIM-1 gene in German livestock is restricted neither to a certain bacterial species nor to a certain Salmonella serovar but is linked to a particular Tn21-like transposable element located on transferable pSE15-SA01028-like IncHI2 (ST1) plasmids, being present in all of the investigated isolates from 2011 to 2017.IMPORTANCE Carbapenems are considered one of few remaining treatment options against multidrug-resistant Gram-negative pathogens in human clinical settings. The occurrence of carbapenemase-producing Enterobacteriaceae in livestock and food is a major public health concern. Particularly the occurrence of VIM-1-producing Salmonella Infantis in livestock farms is worrisome, as this zoonotic pathogen is one of the main causes for human salmonellosis in Europe. Investigations on the epidemiology of those carbapenemase-producing isolates and associated mobile genetic elements through an in-depth molecular characterization are indispensable to understand the transmission of carbapenemase-producing Enterobacteriaceae along the food chain and between different populations to develop strategies to prevent their further spread.

Interplay between Peptidoglycan Biology and Virulence in Gram-Negative Pathogens

The clinical and epidemiological threat of the growing antimicrobial resistance in Gram-negative pathogens, particularly for β-lactams, the most frequently used and relevant antibiotics, urges research to find new therapeutic weapons to combat the infections caused by these microorganisms. An essential previous step in the development of these therapeutic solutions is to identify their potential targets in the biology of the pathogen. This is precisely what we sought to do in this review specifically regarding the barely exploited field analyzing the interplay among the biology of the peptidoglycan and related processes, such as β-lactamase regulation and virulence. Hence, here we gather, analyze, and integrate the knowledge derived from published works that provide information on the topic, starting with those dealing with the historically neglected essential role of the Gram-negative peptidoglycan in virulence, including structural, biogenesis, remodeling, and recycling aspects, in addition to proinflammatory and other interactions with the host. We also review the complex link between intrinsic β-lactamase production and peptidoglycan metabolism, as well as the biological costs potentially associated with the expression of horizontally acquired β-lactamases. Finally, we analyze the existing evidence from multiple perspectives to provide useful clues for identifying targets enabling the future development of therapeutic options attacking the peptidoglycan-virulence interconnection as a key weak point of the Gram-negative pathogens to be used, if not to kill the bacteria, to mitigate their capacity to produce severe infections.

Molecular Characterization of Carbapenem-Resistant Acinetobacter baumannii in the Intensive Care Unit of Uruguay's University Hospital Identifies the First rmtC Gene in the Species

Carbapenem-resistant Acinetobacter baumannii (CRAB) infections are an increasing concern in intensive care units (ICUs) worldwide. The combination of carbapenemases and 16S rRNA-methyltransferases (16S-RMTases) further reduces the therapeutic options. OXA-carbapenemase/A. baumannii clone tandems in Latin America have already been described; however, no information exists in this region regarding the occurrence of 16S-RMTases in this microorganism. In addition, the epidemiology of A. baumannii in ICUs and its associated resistance profiles are poorly understood. Our objectives were as follows: to study the clonal relationship and antibiotic resistance profiles of clinical and digestive colonizing A. baumannii isolates in an ICU, to characterize the circulating carbapenemases, and to detect 16S-RMTases. Patients admitted between August 2010 and July 2011 with a clinically predicted hospital stay > 48 hr were included. Pharyngeal and rectal swabs were obtained during the first fortnight after hospitalization. Resistance profiles were determined with MicroScan^® and VITEK2 system. Carbapenemases and 16S-RMTases were identified by PCR and sequencing, and clonality was assessed by pulsed-field gel electrophoresis and multilocus sequence typing. Sixty-nine patients were studied and 63 were diagnosed with bacterial infections. Among these, 29 were CRAB isolates; 49 A. baumannii were isolated as digestive colonizers. These 78 isolates were clustered in 7 pulsetypes, mostly belonging to ST79. The only carbapenemase genes detected were bla_OXA-51 (n = 78), bla_OXA-23 (n = 62), and bla_OXA-58 (n = 3). Interestingly, two clinical isolates harbored the rmtC 16S-RMTase gene. To the best of our knowledge, this is the first description of the presence of rmtC in A. baumannii.

Diversity and regulation of intrinsic β-lactamases from non-fermenting and other Gram-negative opportunistic pathogens

This review deeply addresses for the first time the diversity, regulation and mechanisms leading to mutational overexpression of intrinsic β-lactamases from non-fermenting and other non-Enterobacteriaceae Gram-negative opportunistic pathogens. After a general overview of the intrinsic β-lactamases described so far in these microorganisms, including circa. 60 species and 100 different enzymes, we review the wide array of regulatory pathways of these β-lactamases. They include diverse LysR-type regulators, which control the expression of β-lactamases from relevant nosocomial pathogens such as Pseudomonas aeruginosa or Stenothrophomonas maltophilia or two-component regulators, with special relevance in Aeromonas spp., along with other pathways. Likewise, the multiple mutational mechanisms leading to β-lactamase overexpression and β-lactam resistance development, including AmpD (N-acetyl-muramyl-L-alanine amidase), DacB (PBP4), MrcA (PPBP1A) and other PBPs, BlrAB (two-component regulator) or several lytic transglycosylases among others, are also described. Moreover, we address the growing evidence of a major interplay between β-lactamase regulation, peptidoglycan metabolism and virulence. Finally, we analyse recent works showing that blocking of peptidoglycan recycling (such as inhibition of NagZ or AmpG) might be useful to prevent and revert β-lactam resistance. Altogether, the provided information and the identified gaps should be valuable for guiding future strategies for combating multidrug-resistant Gram-negative pathogens.

Competition assays between ESBL-producing E. coli and K. pneumoniae isolates collected from Lebanese elderly: An additional cost on fitness

The dissemination of Multi Drug Resistant Organisms (MDROs) is one of the major public health problems addressed nowadays. High fecal carriage rates of MDR Enterobacteriaceae were reported from Lebanese nursing homes. Studies have shown that the acquisition of resistance genes by bacteria might confer a fitness cost detected as a decrease in the frequency of these bacteria as compared to sensitive isolates. In this study, the competitive growth of MDR Enterobacteriaceae isolated from elderly is assessed. Sensitive and ESBL-producing Escherichia coli and Klebsiella pneumoniae isolates were identified. Inter-species in-vitro competition assays were conducted in different combinations. ESBL-producing K. pneumoniae presented a fitness cost when competing against sensitive E. coli. On the other hand, resistant E. coli only showed a fitness cost when growing in presence of two sensitive K. pneumoniae isolates. These results suggest that ESBL-production genes in E. coli and K. pneumoniae may confer a fitness cost that leads to the decrease in frequency of these bacteria in interspecies competitions. Culturing bacteria in a medium with more diverse isolates can provide better insights into bacterial competition and resistance dynamics, which can be exploited in the search for alternative therapeutic approaches towards the colonization of resistant bacteria.

Analysis of plasmid-mediated quinolone and oxyimino-cephalosporin resistance mechanisms in Uruguayan Salmonella enterica isolates from 2011-2013

This study characterised the mechanisms of fluoroquinolone and oxyimino-cephalosporin resistance in human Salmonella enterica isolates in Uruguay. Salmonella enterica isolates were collected from 2011-2013 and were selected based on non-susceptibility to ciprofloxacin and/or oxyimino-cephalosporins. The disk diffusion assay was performed for various antibiotics, and the ciprofloxacin minimum inhibitory concentration (MIC) was determined following CLSI guidelines. Genetic relatedness was determined following PulseNet protocols. Extended-spectrum β-lactamases, ampC alleles and plasmid-mediated quinolone resistance were characterised by PCR and sequencing. Plasmid analyses were carried out by conjugation or transformation assays, and plasmid-encoded genes were identified by PCR. Mutations in the quinolone resistance-determining region of gyrases were sought by PCR and sequencing. Among 579 isolates, 105 (18.4%) ciprofloxacin-non-susceptible (CIP-NS) isolates, 9 (1.6%) oxyimino-cephalosporin-resistant isolates and 2 (0.3%) isolates resistant to both antibiotic families were detected. Thirteen isolates carried qnrB alleles (twelve qnrB19 and one qnrB2), four carried blaCTX-M-8, two blaCTX-M-14, two blaSHV-2 and three blaCMY-2-like genes. No correlation was found between mutations in gyrases and ciprofloxacin MICs. Several co-circulating clones of S. enterica ssp. enterica serovar Typhimurium were detected; conversely, S. enterica ssp. enterica serovar Enteritidis corresponded mainly to a single circulating clone. Nine (75%) of twelve of CIP-NS extraintestinal isolates shared the same pulsotype with intestinal isolates. During the study period, the frequency of CIP-NS isolates increased, albeit with ciprofloxacin MICs of 0.125-0.5mg/L. Detection of the same quinolone-resistant clones recovered both from intestinal and extraintestinal samples highlights the significance of epidemiological surveillance of antibiotic susceptibility for every human Salmonella isolate.