Understanding the Relationship Between Antiviral Prescription Data and COVID-19 Incidence in New York City: A Retrospective Cohort Study

The coronavirus disease 2019 (COVID-19) pandemic has caused more than 675 million confirmed cases and nearly 7 million deaths worldwide [1]. While testing for COVID-19 was initially centered in health care facilities, with required reporting to health departments, it is increasingly being performed in the home with rapid antigen testing [2]. Most at-home tests are self-interpreted and not reported to a provider or health department, which could lead to delayed reporting or underreporting of cases [3]. As such, there is a strong possibility that reported cases may become a less reliable indicator of transmission over time.

1901. Utilizing Antiviral Prescription Data as a Predictor of Increasing COVID-19 Incidence: A Retrospective Study of Citywide Data

Background

COVID-19 has caused over 6 million deaths worldwide. While testing was initially centered in healthcare facilities with automatic reporting to health departments, it is increasingly being performed in the home with rapid antigen testing. Many at-home tests are self-interpreted and not typically reported.

In December 2021, nirmatrelvir/ritonavir and molnupiravir were authorized for the treatment of COVID-19 in high-risk patients. In New York City (NYC), these antivirals are available for free via pickup or home delivery, allowing for early expanded access. We sought to identify if prescribing of antivirals might act as an early indicator of increasing COVID-19 incidence in the setting of decreased case reporting.

Methods

Confirmed and probable COVID-19 tests are reported to the NYC Health Department. All pharmacies participating in the federal COVID-19 therapeutics program are required to report daily utilization data, which are made available to jurisdictions through the Tiberius platform. From December 27, 2021 to April 18, 2022, we analyzed citywide weekly case counts, using patients 65 years and over as a proxy for high-risk eligible patients, and weekly oral therapeutic prescription fills. We calculated weekly case rates and prescribing rates per 100,000 residents. Weekly case rates and prescription fill rates were trended to assess for concordance over time.

Results

During this period, 17,522 prescriptions were filled. Prescription fills and case rates were concordant until the weeks of February 28 and March 7, when an increase in fill rate was noted from the week prior (1.76 to 2.34 to 4.10), while reported case incidence continued to decrease (6.16 to 5.08 to 4.61). This discordance coincided with the emergence of the BA.2 subvariant in NYC. After these weeks, an uptick in case incidence was noted for the remainder of the study period, in concordance with prescription data.

Conclusion

Increasing rates of oral therapeutic prescribing could be an early indicator of increasing COVID-19 transmission. Limitations include the unclear impact of public and prescriber education efforts on prescribing trends. Next steps include exploring the addition of prescription data into modeling to predict trends in transmission as reported cases becomes a less reliable indicator.

Disclosures

Gabriel M. Cohen, MD, Daybreak Health: Advisor/Consultant|Daybreak Health: Board Member|Daybreak Health: Ownership Interest Andrew B. Wallach, MD, Pfizer: Advisor/Consultant.

Serum Antibody Responses against Carbapenem-Resistant Klebsiella pneumoniae in Infected Patients

Capsular polysaccharide (CPS) heterogeneity within carbapenem-resistant Klebsiella pneumoniae (CR-Kp) strain sequence type 258 (ST258) must be considered when developing CPS-based vaccines. Here, we sought to characterize CPS-specific antibody responses elicited by CR-Kp-infected patients. Plasma and bacterial isolates were collected from 33 hospital patients with positive CR-Kp cultures. Isolate capsules were typed by wzi sequencing. Reactivity and measures of efficacy of patient antibodies were studied against 3 prevalent CR-Kp CPS types (wzi29, wzi154, and wzi50). High IgG titers against wzi154 and wzi50 CPS were documented in 79% of infected patients. Patient-derived (PD) IgGs agglutinated CR-Kp and limited growth better than naive IgG and promoted phagocytosis of strains across the serotype isolated from their donors. Additionally, poly-IgG from wzi50 and wzi154 patients promoted phagocytosis of nonconcordant CR-Kp serotypes. Such effects were lost when poly-IgG was depleted of CPS-specific IgG. Additionally, mice infected with wzi50, wzi154, and wzi29 CR-Kp strains preopsonized with wzi50 patient-derived IgG exhibited lower lung CFU than controls. Depletion of wzi50 antibodies (Abs) reversed this effect in wzi50 and wzi154 infections, whereas wzi154 Ab depletion reduced poly-IgG efficacy against wzi29 CR-Kp We are the first to report cross-reactive properties of CPS-specific Abs from CR-Kp patients through both in vitro and in vivo models.IMPORTANCE Carbapenem-resistant Klebsiella pneumoniae is a rapidly emerging public health threat that can cause fatal infections in up to 50% of affected patients. Due to its resistance to nearly all antimicrobials, development of alternate therapies like antibodies and vaccines is urgently needed. Capsular polysaccharides constitute important targets, as they are crucial for Klebsiella pneumoniae pathogenesis. Capsular polysaccharides are very diverse and, therefore, studying the host's capsule-type specific antibodies is crucial to develop effective anti-CPS immunotherapies. In this study, we are the first to characterize humoral responses in infected patients against carbapenem-resistant Klebsiella pneumoniae expressing different wzi capsule types. This study is the first to report the efficacy of cross-reactive properties of CPS-specific Abs in both in vitro and in vivo models.

CRISPR-Cas influences the acquisition of antibiotic resistance in Klebsiella pneumoniae

In the US Carbapenem resistance in Klebsiella pneumoniae (Kp) is primarily attributed to the presence of the genes bla_KPC-2 and bla_KPC-3, which are transmitted via plasmids. Carbapenem-resistant Kp (CR-Kp) infections are associated with hospital outbreaks. They are difficult to treat, and associated with high mortality rates prompting studies of how resistance is obtained. In this study, we determined the presence of CRISPR-Cas in 304 clinical Kp strains. The CRISPR-Cas system has been found to prevent the spread of plasmids and bacteriophages, and therefore limits the horizontal gene transfer mediated by these mobile genetic elements. Here, we hypothesized that only those Kp strains that lack CRISPR-Cas can acquire CR plasmids, while those strains that have CRISPR-Cas are protected from gaining these plasmids and thus maintain sensitivity to antimicrobials. Our results show that CRISPR-Cas is absent in most clinical Kp strains including the clinically important ST258 clone. ST258 strains that continue to be sensitive to carbapenems also lack CRISPR-Cas. Interestingly, CRISPR-Cas positive strains, all non-ST258, exhibit lower resistance rates to antimicrobials than CRISPR-Cas negative strains. Importantly, we demonstrate that the presence of CRISPR-Cas appears to inhibit the acquisition of bla_KPC plasmids in 7 Kp strains. Furthermore, we show that strains that are unable to acquire bla_KPC plasmids contain CRISPR spacer sequences highly identical to those found in previously published multidrug-resistance-containing plasmids. Lastly, to our knowledge this is the first paper demonstrating that resistance to bla_KPC plasmid invasion in a CRISPR-containing Kp strain can be reversed by deleting the CRISPR-cas cassette.

783. Infectious complications in IV abusers: a detailed review of hospitalized IV abusers

Background

With 337 heroin-related deaths between 2009 and 2013, Suffolk County reported more such deaths than any other county in New York State. Suffolk County’s population of 1.5 million is larger than that of some states. Compared with the rest of New York State, median annual household income is the third highest of New York’s 62 counties and the income gap between socioeconomic classes is smaller.

Methods

Detailed retrospective chart review was performed on admitted patients to Stony Brook University Hospital (SBUH) from November 2015 to October 2018 with active IV drug abuse and infection. Demographics, drug use characteristics, microbiology, co-morbidities, and outcome were assessed.

Results

198 active IV heroin users were admitted with a complicated infection during this time frame. Most patients were Caucasian (94%) and more were male (64%). The median age was 33 (range 17 to 58). Most patients (90%) had health insurance, many lived with others at home and were unemployed. Thirty-three were on methadone and 28 on suboxone. The median length of stay was 4.7 days (ranging from 1 to 140 days) and 29% of admitted patients left against medical advice. 60% of the patients had been admitted to a hospital in the prior 12 months. The majority (58%) presented with cellulitis, 12% were diagnosed with endocarditis, 10% with osteomyelitis, 6.5% with bacteremia/sepsis. Ten were also pregnant. Infectious disease was consulted in 55%, and Psychiatry and social worker in 46% of cases. HIV, HepC and HBV testing were not performed in less than 50% of patients. There was a total of 5 deaths during the hospitalization, 2 patients with endocarditis, 2 with sepsis and one overdose. 45% of patients with endocarditis and 43% of patients with osteomyelitis required surgical intervention. S. aureus was a common pathogen and identified in 61% of the cases, where a pathogen was isolated.

Conclusion

Our data show that even in an affluent community opioid addiction is not properly addressed during hospital admissions. As a consequence, users sign out against medical advice, have frequent hospital admissions, remain out of addiction treatment, and outcome is poor. A systematic multidisciplinary approach will be required to improve the care for this vulnerable patient population

Disclosures

All authors: No reported disclosures.

405. Serum Antibody Responses Against Carbapenem-Resistant Klebsiella pneumoniae in Infected Patients

Background

Capsular polysaccharide (CPS) of Carbapenem-resistant K. pneumoniae ST258 (CR-Kp) is a potential vaccine target. CPS of these isolates generally falls within 2 homology groups named clade 1 and clade 2. We and others have made antibodies (Abs) that act against clade2 CR-Kp but failed to make therapeutic Abs against clade1 CR-Kp. Previous studies had shown that studying patient’s antibody responses could help in identifying suitable candidates for developing immunotherapies. Thus, we sought to identify potential vaccine candidates by investigating the humoral response CPS in CR-Kp-infected patients.

Methods

24 CR-Kp isolates and corresponding serums were collected from inpatients at Stony Brook Hospital. CPS was isolated and purified by size-exclusion column chromatography from CR-Kp strains 34 (clade 2), 36 (clade 1), and 38 (clade-Other). Anti-CPS Abs in patient’s serum were detected by enzyme-linked immunosorbent assay (ELISA) and bulk Abs from positive serum were purified using an affinity column. These Abs were tested for activity against CR-Kp by serum bactericidal and agglutination assays.

Results

50% of clade2 CR-Kp-infected patients had humoral responses against CPS34. 77% of clade 1-infected patients sera cross-reacted wtih CPS34, but none of them developed Abs against CPS36. Interestingly, 90% of clade1 and 60% of clade 2-infected patients, respectively, showed Abs binding to CPS38. Thus, we selectively purified Anti-CPS Abs from two clade-Other-infected patients and observed that they were cross-reactive with all three CPS. Further, these Anti-CPS Abs agglutinated all tested CR-Kp isolates (34, 36, and 38) when compared with control human IgG (P < 0.005). Additionally, these Anti-CPS Abs promoted killing of clade2 bacteria and inhibited the growth of clade1 bacteria in Ab-mediated serum bactericidal assay. These data elucidate that humoral responses developed in clade-Other CR-Kp-infected patients have therapeutic potential.

Conclusion

With the unavailability of effective antimicrobials for CR-Kp, approaches like developing novel anti-CPS vaccine could serve as an alternate therapy. Our data suggest that developing immunotherapies targeting CPS38 could potentially provide protection across both clade1 and clade2 bacteria in clinical settings.

Disclosures

All authors: No reported disclosures.

621. In vitro Ceftazidime: Avibactam Resistance in Carbapenem-Resistant Enterobacteriaceae Isolates

Background

Ceftazidime–avibactam (CAZ-AVI) is a new antibiotic with activity against many Carbapenem-resistant Enterobacteriaceae (CRE). Although CAZ-AVI resistance in CRE has been reported, it is not consistently assessed. Our study aimed to assess the prevalence of CAZ-AVI resistance in CRE isolated from patients with and without prior exposure to CAZ-AVI.

Methods

We tested 116 CRE isolates for CAZ-AVI resistance by Kirby–Bauer (KB) disk diffusion susceptibility. Resistant isolates were verified by repeat KB and E-test performed by the Stony Brook Hospital laboratory. The bla_KPC gene of resistant strains was amplified by PCR and sequenced. Patient data were used to determine whether patients were colonized or infected, and whether they were exposed to CAZ-AVI.

Results

Of the 116 CRE isolates from 86 patients (96 encounters), 50% were Klebsiella species, 23.2% were Enterobacter species, 10.3% Escherichia coli and 16.5% other CRE. They were recovered from colonized (37%) and infected (63%) patients of which 18% were treated with CAZ-AVI during their hospitalizations (median duration of therapy, 6 days). Two CRE isolates (1.7%) were found to be resistant on repeated testing. One isolate was K. pneumoniae derived from the sputum of a patient diagnosed with ventilator-associated pneumonia who received 40 days of CAZ-AVI therapy prior to isolation of the resistant isolate (KB diameter 20 mm, MIC > 512 μg/mL by E-Test). Sequencing of the strain’s bla_KPC3 gene revealed a previously described Ambler-position D179Y mutation that has been shown to convey resistance. The second CAZ-AVI-resistant K. pneumoniae (KB diameter 19 mm, MIC 64 μg/mL by E-test) was isolated from the urine of a colonized patient naïve to CAZ-AVI therapy. The strain’s bla_KPC10 gene had no mutations.

Conclusion

In our strain collection, the rate of resistance to CAZ-AVI remains low <2%. Although we found one mutation (D179Y) previously linked to CAZ-AVI resistance we also discovered one K. pneumoniae isolate with in vitro resistance to CAZ-AVI that did not exhibit any bla_KPC mutations conveying CAZ-AVI resistance. Interestingly, this strain was derived from a patient with no prior CAZ-AVI exposure. Whole-genome sequencing will be performed to identify other genes or mutations that may confer resistance.

Disclosures

All authors: No reported disclosures.

Pentoxifylline Alone or in Combination with Gentamicin or Vancomycin Inhibits Live Microbe-Induced Proinflammatory Cytokine Production in Human Cord Blood and Cord Blood Monocytes In Vitro

Neonatal sepsis and its accompanying inflammatory response contribute to substantial morbidity and mortality. Pentoxifylline (PTX), a phosphodiesterase inhibitor which suppresses transcription and production of proinflammatory cytokines, is a candidate adjunctive therapy for newborn sepsis. We hypothesized that PTX decreases live microbe-induced inflammatory cytokine production in newborn blood. Cord blood was stimulated with live microorganisms commonly encountered in newborn sepsis (Escherichia coli, Staphylococcus aureus, Staphylococcus epidermidis, or Candida albicans) and simultaneously treated with antimicrobial agents (gentamicin, vancomycin, or amphotericin B) and/or clinically relevant concentrations of PTX. Microbial colony counts were enumerated by plating, supernatant cytokines were measured by multiplex assay, intracellular cytokines and signaling molecules were measured by flow cytometry, and mRNA levels were measured by quantitative reverse transcription-PCR. PTX inhibited concentration-dependent E. coli-, S. aureus-, S. epidermidis-, and C. albicans-induced tumor necrosis factor (TNF) and E. coli-induced interleukin-1β (IL-1β) production in whole blood, with greater suppression of proinflammatory cytokines in combination with antimicrobial agents. Likewise, PTX suppressed E. coli-induced monocytic TNF and IL-1β, whereby combined PTX and gentamicin led to significantly greater reduction of TNF and IL-1β. The anti-inflammatory effect of PTX on microbe-induced proinflammatory cytokine production was accompanied by inhibition of TNF mRNA expression and was achieved without suppressing the production of the anti-inflammatory IL-10. Of note, microbial colony counts in newborn blood were not increased by PTX. Our findings demonstrated that PTX inhibited microbe-induced proinflammatory cytokine production, especially when combined with antimicrobial agents, without enhancing microbial proliferation in human cord blood in vitro, thus supporting its utility as candidate adjunctive agent for newborn sepsis.

1196. Serratia marcescens Strains Carrying blaKPC-2 and blaKPC-3 Carbapenemase Associated With Chronic Mechanical Ventilation

Background

Carbapenem resistance (CR) in Enterobacteriaceae is a growing concern which the CDC has designated as an urgent threat. At our institution we have noted emergence of CR strains in clinical isolates including a growing number of Serratia marcescens. CR in Serratia marcescens in the United States is mostly reported to be encoded by the SME family of chromosomally encoded carbapenemases, while in Asia it has been described being mediated by transmissible plasmids such as KPC. Here we describe the emergence and characteristics of CR Serratia marcescens at an academic tertiary care hospital in New York.

Methods

Serratia marcescens isolates demonstrating in vitro carbapenem resistance were recovered over a 12-month period from six distinct patients. Antibiotic resistance was determined by standard methods. Real-time PCR for bla(KPC), mcr gene, bla(NDM-1), bla(VIM), and bla(OXA-48) was performed. Patient comorbidities, source of culture, location in the hospital, and co-infection with other CR organisms were investigated.

Results

Fourteen S. marcescens isolates demonstrating in vitro carbapenem resistance were recovered from six individual patients. All six patients had a history of chronic respiratory failure with tracheostomy and at least partial ventilator dependence. Five of the patients were located on the pulmonary intermediate care unit, and one in the pediatric intensive care unit. Twelve of the 14 isolates were tracheal or sputum cultures. Five of the sputum cultures from two patients were co-infected with CR Pseudomonas, and one sputum culture was simultaneously positive for CR Klebsiella pneumoniae and Enterobacter Cloacae. Nine out of 14 isolates were positive for bla^KPC-3, two were bla^KPC-2 positive, and three were bla^KPC-negative, with no mechanism of carbapenem resistance determined yet. None of the other genes were detected.

Conclusion

Most carbapenem-resistant Serratia isolates were derived from respiratory tract and were found to be positive for bla^KPC-3. This suggests that plasmid encoded carbapenemases are emerging among Serratia in the United States, which is already being reported in China. Genomic sequencing may establish whether this represents a clonal expansion and whether the bla^KPC plasmid was transferred from Klebsiella or Enterobacter to Serratia in one of the patients.

Disclosures

All authors: No reported disclosures.

A Semi-Synthetic Glycoconjugate Vaccine Candidate for Carbapenem-Resistant Klebsiella pneumoniae

Hospital-acquired infections are an increasingly serious health concern. Infections caused by carpabenem-resistant Klebsiella pneumoniae (CR-Kp) are especially problematic, with a 50 % average survival rate. CR-Kp are isolated from patients with ever greater frequency, 7 % within the EU but 62 % in Greece. At a time when antibiotics are becoming less effective, no vaccines to protect from this severe bacterial infection exist. Herein, we describe the convergent [3+3] synthesis of the hexasaccharide repeating unit from its capsular polysaccharide and related sequences. Immunization with the synthetic hexasaccharide 1 glycoconjugate resulted in high titers of cross-reactive antibodies against CR-Kp CPS in mice and rabbits. Whole-cell ELISA was used to establish the surface staining of CR-Kp strains. The antibodies raised were found to promote phagocytosis. Thus, this semi-synthetic glycoconjugate is a lead for the development of a vaccine against a rapidly progressing, deadly bacterium.

Generational distribution of a Candida glabrata population: Resilient old cells prevail, while younger cells dominate in the vulnerable host

Similar to other yeasts, the human pathogen Candida glabrata ages when it undergoes asymmetric, finite cell divisions, which determines its replicative lifespan. We sought to investigate if and how aging changes resilience of C. glabrata populations in the host environment. Our data demonstrate that old C. glabrata are more resistant to hydrogen peroxide and neutrophil killing, whereas young cells adhere better to epithelial cell layers. Consequently, virulence of old compared to younger C. glabrata cells is enhanced in the Galleria mellonella infection model. Electron microscopy images of old C. glabrata cells indicate a marked increase in cell wall thickness. Comparison of transcriptomes of old and young C. glabrata cells reveals differential regulation of ergosterol and Hog pathway associated genes as well as adhesion proteins, and suggests that aging is accompanied by remodeling of the fungal cell wall. Biochemical analysis supports this conclusion as older cells exhibit a qualitatively different lipid composition, leading to the observed increased emergence of fluconazole resistance when grown in the presence of fluconazole selection pressure. Older C. glabrata cells accumulate during murine and human infection, which is statistically unlikely without very strong selection. Therefore, we tested the hypothesis that neutrophils constitute the predominant selection pressure in vivo. When we altered experimentally the selection pressure by antibody-mediated removal of neutrophils, we observed a significantly younger pathogen population in mice. Mathematical modeling confirmed that differential selection of older cells is sufficient to cause the observed demographic shift in the fungal population. Hence our data support the concept that pathogenesis is affected by the generational age distribution of the infecting C. glabrata population in a host. We conclude that replicative aging constitutes an emerging trait, which is selected by the host and may even play an unanticipated role in the transition from a commensal to a pathogen state.

Carbapenem-resistant Klebsiella pneumoniae exhibit variability in capsular polysaccharide and capsule associated virulence traits

BACKGROUND

Novel therapies are urgently needed to treat carbapenem-resistant Klebsiella pneumoniae (CR-Kp)-mediated infection, which constitute a major health threat in the United States. In order to assess if it is feasible to develop anticapsular antibodies as a potential novel therapy, it is crucial to first systematically characterize capsular polysaccharide (CPS) and virulence traits in these strains.

METHODS

Forty CR-Kp were genotyped by pulsed field gel electrophoresis, multilocus sequence typing (MLST), and molecular capsule typing (C-patterns and wzi sequencing). Their biofilm formation, serum resistance, macrophage-mediated killing, and virulence in Galleria mellonella were compared. MAb (1C9) was generated by co-immunization with 2 CPSs, and cross-reactivity was investigated.

RESULTS

MLST assigned 80% of CR-Kp isolates to the ST258-clone. Molecular capsule typing identified new C-patterns, including C200/wzi-154, which was widely represented and associated with blaKPC-3-bearing strains. Heterogeneity was detected in biofilm formation and macrophage-mediated killing. Differences in serum resistance correlated with virulence in G. mellonella. ST258 strains carrying blaKPC-3 were less virulent than those with blaKPC-2. MAb 1C9 cross-reacted with 58% of CR-Kp CPSs.

CONCLUSIONS

CR-Kp ST258 strains exhibit variability of virulence-associated traits. Differences were associated with the type of KPC gene and CPS. Identification of cross-reacting anti-CPS mAbs encourages their development as adjunctive therapy.

A common origin for the bacterial toxin-antitoxin systems parD and ccd, suggested by analyses of toxin/target and toxin/antitoxin interactions

Bacterial toxin-antitoxin (TA) systems encode two proteins, a potent inhibitor of cell proliferation (toxin) and its specific antidote (antitoxin). Structural data has revealed striking similarities between the two model TA toxins CcdB, a DNA gyrase inhibitor encoded by the ccd system of plasmid F, and Kid, a site-specific endoribonuclease encoded by the parD system of plasmid R1. While a common structural fold seemed at odds with the two clearly different modes of action of these toxins, the possibility of functional crosstalk between the parD and ccd systems, which would further point to their common evolutionary origin, has not been documented. Here, we show that the cleavage of RNA and the inhibition of protein synthesis by the Kid toxin, two activities that are specifically counteracted by its cognate Kis antitoxin, are altered, but not inhibited, by the CcdA antitoxin. In addition, Kis was able to inhibit the stimulation of DNA gyrase-mediated cleavage of DNA by CcdB, albeit less efficiently than CcdA. We further show that physical interactions between the toxins and antitoxins of the different systems do occur and define the stoichiometry of the complexes formed. We found that CcdB did not degrade RNA nor did Kid have any reproducible effect on the tested DNA gyrase activities, suggesting that these toxins evolved to reach different, rather than common, cellular targets.

The toxin-antitoxin proteins relBE2Spn of Streptococcus pneumoniae: characterization and association to their DNA target

The chromosome of the pathogenic Gram-positive bacterium Streptococcus pneumoniae contains between six to 10 operons encoding toxin-antitoxin systems (TAS). TAS are widespread and redundant in bacteria and archaea and their role, albeit still obscure, may be related to important aspects of bacteria lifestyle like response to stress. One of the most abundant TAS is the relBE family, being present in the chromosome of many bacteria and archaea. Because of the high rates of morbility and mortality caused by S. pneumoniae, it has been interesting to gain knowledge on the pneumococcal TAS, among them the RelBE2Spn proteins. Here, we have analyzed the DNA binding capacity of the RelB2Spn antitoxin and the RelB2Spn-RelE2Spn proteins by band-shift assays. Thus, a DNA region encompassing the operator region of the proteins was identified. In addition, we have used analytical ultracentrifugation and native mass spectrometry to measure the oligomerization state of the antitoxin alone and the RelBE2Spn complex in solution bound or unbound to its DNA substrate. Using native mass spectrometry allowed us to unambiguously determine the stoichiometry of the RelB2Spn and of the RelBE2Spn complex alone or associated to its DNA target.

Kis antitoxin couples plasmid R1 replication and parD (kis,kid) maintenance modules

The coupling between the replication and parD (kis, kid) maintenance modules of R1 has been revisited here by the isolation of a significant collection of conditional replication mutants in the pKN1562 mini-R1 plasmid, and in its derivative, pJLV01, specifically affected in the RNase activity of the Kid toxin. This new analysis aims to identify key factors in this coupling. For this purpose we have quantified and characterized the restriction introduced by parD to isolate conditional replication mutants of this plasmid, a signature of the modular coupling. This restriction depends on the RNase activity of the Kid toxin and it is relieved by either over-expression of the Kis antitoxin or by preventing its degradation by Lon and ClpAP proteases. Based on these data and on the correlation between copy numbers and parD transcriptional levels obtained in the different mutants, it is proposed that a reduction of Kis antitoxin levels in response to inefficient plasmid replication is the key factor for coupling plasmid replication and parD modules.

A mutagenic analysis of the RNase mechanism of the bacterial Kid toxin by mass spectrometry

Kid, the toxin of the parD (kis, kid) maintenance system of plasmid R1, is an endoribonuclease that preferentially cleaves RNA at the 5' of A in the core sequence 5'-UA(A/C)-3'. A model of the Kid toxin interacting with the uncleavable mimetic 5'-AdUACA-3' is available. To evaluate this model, a significant collection of mutants in some of the key residues proposed to be involved in RNA binding (T46, A55, T69 and R85) or RNA cleavage (R73, D75 and H17) were analysed by mass spectrometry in RNA binding and cleavage assays. A pair of substrates, 5'-AUACA-3', and its uncleavable mimetic 5'-AdUACA-3', used to establish the model and structure of the Kid-RNA complex, were used in both the RNA cleavage and binding assays. A second RNA substrate, 5'-UUACU-3' efficiently cleaved by Kid both in vivo and in vitro, was also used in the cleavage assays. Compared with the wild-type protein, mutations in the residues of the catalytic site abolished RNA cleavage without substantially altering RNA binding. Mutations in residues proposed to be involved in RNA binding show reduced binding efficiency and a corresponding decrease in RNA cleavage efficiency. The cleavage profiles of the different mutants were similar with the two substrates used, but RNA cleavage required much lower protein concentrations when the 5'-UUACU-3' substrate was used. Protein synthesis and growth assays are consistent with there being a correlation between the RNase activity of Kid and its inhibitory potential. These results give important support to the available models of Kid RNase and the Kid-RNA complex.

Novel Escherichia coli RF1 mutants with decreased translation termination activity and increased sensitivity to the cytotoxic effect of the bacterial toxins Kid and RelE

Novel mutations in prfA, the gene for the polypeptide release factor RF1 of Escherichia coli, were isolated using a positive genetic screen based on the parD (kis, kid) toxin–antitoxin system. This original approach allowed the direct selection of mutants with altered translational termination efficiency at UAG codons. The isolated prfA mutants displayed a ∼10-fold decrease in UAG termination efficiency with no significant changes in RF1 stability in vivo. All three mutations, G121S, G301S and R303H, were situated close to the nonsense codon recognition site in RF1:ribosome complexes. The prfA mutants displayed increased sensitivity to the RelE toxin encoded by the relBE system of E. coli, thus providing in vivo support for the functional interaction between RF1 and RelE. The prfA mutants also showed increased sensitivity to the Kid toxin. Since this toxin can cleave RNA in a ribosome-independent manner, this result was not anticipated and provided first evidence for the involvement of RF1 in the pathway of Kid toxicity. The sensitivity of the prfA mutants to RelE and Kid was restored to normal levels upon overproduction of the wild-type RF1 protein. We discuss these results and their utility for the design of novel antibacterial strategies in the light of the recently reported structure of ribosome-bound RF1.