Effect of parenteral fluoroquinolone administration on persistence of vancomycin-resistant Enterococcus faecium in the mouse gastrointestinal tract

Effect of Intramuscularly Administered Oxytetracycline or Enrofloxacin on Vancomycin-Resistant Enterococci, Extended Spectrum Beta-Lactamase- and Carbapenemase-Producing Enterobacteriaceae in Pigs

Simple Summary

Nowadays, there is great concern about the prevalence of multidrug resistant bacteria in food-producing animals since they are potential sources of transmission to humans. The aim of this work was to evaluate the effect of two antibiotics (oxytetracycline and enrofloxacin) treatments in pigs on resistant bacteria that are considered a threat to public health. This study highlights that the use of oxytetracycline or enrofloxacin in food-producing animals could select resistant bacteria in pig faeces. Special care should be taken to avoid faecal contamination of carcasses during slaughter.

Abstract

Nowadays, there is a great concern about the prevalence of multidrug resistant Enterococcus spp. and Enterobacteriaceae in food-producing animals. The aim of this work was to evaluate the effect of oxytetracycline or enrofloxacin treatment on vancomycin-resistant enterococci (VRE), extended spectrum β-lactamase (ESBL) and carbapenemase-producing Enterobacteriaceae in pigs. A total of 26 piglets were received and distributed in three groups. Group 1 was treated with enrofloxacin (N = 12), group 2 with oxytetracycline (N = 10) and group 3 did not receive any treatment (control group) (N = 4). A higher number of vancomycin-resistant E. faecium were recovered compared to E. faecalis. In the pigs treated with enrofloxacin, vancomycin resistant E. faecium was found in a higher percentage of animals than in the control group. ESBL-producing E. coli was not detected in rectal samples from control animals. However, it was detected in 17–20% of animals treated with oxytetracycline on days 6 to 17 and in 17–50% of the animals treated with enrofloxacin. Carbapenemase-producing E. coli was isolated in animals treated with oxytetracycline, but not in animals treated with enrofloxacin or in the control group. This study highlights that the use of oxytetracycline or enrofloxacin in food-producing animals could select ESBL and carbapenemase-producing E. coli. Further studies shall be needed to validate the results obtained, considering a more robust and extended experimental design.

Evaluation of a Biocide Used in the Biological Isolation and Containment Unit of a Veterinary Teaching Hospital

Hospital-acquired infections (HAIs) are a rising problem worldwide, and the best way of coping with them is through infection tracking and surveillance systems, combined with prevention strategies, namely efficient disinfection protocols, that employ various biocides. However, increasing reports about reductions in biocide susceptibility and the development of cross-resistance to antimicrobials emphasize the need for identifying the factors influencing biocide efficiency. In this study, 29 bacterial isolates (n = 3 E. coli, n = 2 Pseudomonas spp., n = 23 Enterococcus spp., and n = 1 Staphylococcus pseudintermedius), obtained from environmental samples collected from the Biological Isolation and Containment Unit (BICU), of the Veterinary Teaching Hospital of the Faculty of Veterinary Medicine, University of Lisbon, were tested in order to determine their antimicrobial susceptibility to various antibiotics. Thirteen of these isolates were further selected in order to determine their antimicrobial susceptibility to Virkon™ S, with and without the presence of organic matter. Afterward, seven of these isolates were incubated in the presence of sub-lethal concentrations of this formulation and, subsequently, new susceptibility profiles were determined. Fourteen of the 29 isolates (48.3%) were classified as multidrug resistant, all previously identified as enterococci. Concerning Virkon™ S's susceptibility, the Minimal Bactericidal Concentration (MBC) of this biocide regarding all isolates was at least eight times lower than the concentration regularly used, when no organic matter was present. However, when organic matter was added, MBC values rose up to 23 times. After exposure to sub-lethal concentrations of Virkon™ S, four enterococci presented a phenotypical change regarding antimicrobial susceptibility towards gentamicin. Virkon™ S also resulted in higher MBC values, up to 1.5 times, in the presence of low concentrations of organic matter, but no rise in these values was observed in assays without interfering substance. Virkon™ S seemed to be an efficient formulation in eliminating all bacteria isolates isolated from the BICU. However, organic matter could represent a hindrance to this ability, which emphasizes the importance of sanitization before disinfection procedures. The changes seen in antimicrobial susceptibility could be explained by a general stress-induced response promoted by the sub-lethal levels of Virkon™ S. Additionally, when no organic matter was present, a decrease in susceptibility to this biocide seemed to be non-existent.

Effects of Antibiotics on the Intestinal Microbiota of Mice

Studies on human and mouse gastrointestinal microbiota have correlated the composition of the microbiota to a variety of diseases, as well as proved it vital to prevent colonization with resistant bacteria, a phenomenon known as colonization resistance. Antibiotics dramatically modify the gut community and there are examples of how antibiotic usage lead to colonization with resistant bacteria [e.g., dicloxacillin usage selecting for ESBL-producing E. coli carriage], as shown by Hertz et al. Here, we investigated the impact of five antibiotics [cefotaxime, cefuroxime, dicloxacillin, clindamycin, and ciprofloxacin] on the intestinal microbiota in mice. Five different antibiotics were each given to groups of five mice. The intestinal microbiotas were profiled by use of the IS-pro analysis; a 16S–23S rDNA interspace [IS]-region-based profiling method. For the mice receiving dicloxacillin and clindamycin, we observed dramatic shifts in dominating phyla from day 1 to day 5. Of note, diversity increased, but overall bacterial load decreased. For ciprofloxacin, cefotaxime, and cefuroxime there were few overall changes. We speculate that antibiotics with efficacy against the abundant anaerobes in the gut, particularly Bacteroidetes, can in fact be selected for resistant bacteria, disregarding the spectrum of activity.

Protection of gut microbiome from antibiotics: development of a vancomycin-specific adsorbent with high adsorption capacity

The fraction of administered antibiotics that reach the cecum and colon causes dysbiosis of the gut microbiome, resulting in various diseases. Protection of the gut microbiome from antibiotics using antibiotic adsorbents in the cecum and colon is a promising method to overcome this issue. Previously, activated charcoal (AC) has been reported to protect the gut microbiome of host animals. AC is an adsorbent that is widely used to capture toxic compounds and overdosed drugs in the gastrointestinal tract. The specificity of adsorbents for antibiotics is critical to avoid the risk of unexpected side effects caused by nonspecific adsorption of biological compounds in the intestinal fluid, such as bile acids and essential micronutrients. Here, we have developed specific adsorbents for vancomycin (VCM), which is known to cause gut dysbiosis. The adsorbents were composed of polyethyleneglycol-based microparticles (MPs) in which a specific ligand for VCM, D-Ala-D-Ala-OH, was attached via dendrons of D-lysine to raise the content of the ligand in the MPs. The MPs successfully protected Staphylococcus lentus from VCM in vitro because of the adsorption of VCM in the culture media. Pre-administration of MPs to mice reduced the amount of free VCM in the feces to an undetectable level. This treatment minimized the effect of VCM on gut microbiota and provided protection against Clostridioides difficile infection after oral challenge with spores.

Pathogenicity of Enterococci

Enterococci are unusually well adapted for survival and persistence in a variety of adverse environments, including on inanimate surfaces in the hospital environment and at sites of infection. This intrinsic ruggedness undoubtedly played a role in providing opportunities for enterococci to interact with other overtly drug-resistant microbes and acquire additional resistances on mobile elements. The rapid rise of antimicrobial resistance among hospital-adapted enterococci has rendered hospital-acquired infections a leading therapeutic challenge. With about a quarter of a genome of additional DNA conveyed by mobile elements, there are undoubtedly many more properties that have been acquired that help enterococci persist and spread in the hospital setting and cause diseases that have yet to be defined. Much remains to be learned about these ancient and rugged microbes, particularly in the area of pathogenic mechanisms involved with human diseases.

The effect of enrofloxacin on enteric Escherichia coli: Fitting a mathematical model to in vivo data

Antimicrobial drugs administered systemically may cause the emergence and dissemination of antimicrobial resistance among enteric bacteria. To develop logical, research-based recommendations for food animal veterinarians, we must understand how to maximize antimicrobial drug efficacy while minimizing risk of antimicrobial resistance. Our objective is to evaluate the effect of two approved dosing regimens of enrofloxacin (a single high dose or three low doses) on Escherichia coli in cattle. We look specifically at bacteria above and below the epidemiological cutoff (ECOFF), above which the bacteria are likely to have an acquired or mutational resistance to enrofloxacin. We developed a differential equation model for the antimicrobial drug concentrations in plasma and colon, and bacteria populations in the feces. The model was fit to animal data of drug concentrations in the plasma and colon obtained using ultrafiltration probes. Fecal E. coli counts and minimum inhibitory concentrations were measured for the week after receiving the antimicrobial drug. We predict that the antimicrobial susceptibility of the bacteria above the ECOFF pre-treatment strongly affects the composition of the bacteria following treatment. Faster removal of the antimicrobial drugs from the colon throughout the study leads to improved clearance of bacteria above the ECOFF in the low dose regimen. If we assume a fitness cost is associated with bacteria above the ECOFF, the increased fitness costs leads to reduction of bacteria above the ECOFF in the low dose study. These results suggest the initial E. coli susceptibility is a strong indicator of how steers respond to antimicrobial drug treatment.

Changes of resistance rates in Pseudomonas aeruginosa strains are unrelated to antimicrobial consumption in ICU populations with invasive device-related infection

OBJECTIVE

To evaluate the relationship between antipseudomonal antibiotic consumption and each individual drug resistance rate in Pseudomonas aeruginosa strains causing ICU acquired invasive device-related infections (IDRI).

DESIGN

A post hoc analysis was made of the data collected prospectively from the ENVIN-HELICS registry.

SETTING

Intensive Care Units participating in the ENVIN-UCI registry between the years 2007 and 2016 (3-month registry each year).

PATIENTS

Patients admitted for over 24h.

MAIN VARIABLES

Annual linear and nonlinear trends of resistance rates of P. aeruginosa strains identified in IDRI and days of treatment of each antipseudomonal antibiotic family per 1000 occupied ICU bed days (DOT) were calculated.

RESULTS

A total of 15,095 episodes of IDRI were diagnosed in 11,652 patients (6.2% out of a total of 187,100). Pseudomonas aeruginosa was identified in 2095 (13.6%) of 15,432 pathogens causing IDRI. Resistance increased significantly over the study period for piperacillin-tazobactam (P<0.001), imipenem (P=0.016), meropenem (P=0.004), ceftazidime (P=0.005) and cefepime (P=0.015), while variations in resistance rates for amikacin, ciprofloxacin, levofloxacin and colistin proved nonsignificant. A significant DOT decrease was observed for aminoglycosides (P<0.001), cephalosporins (P<0.001), quinolones (P<0.001) and carbapenems (P<0.001).

CONCLUSIONS

No significant association was observed between consumption of each antipseudomonal antibiotic family and the respective resistance rates for P. aeruginosa strains identified in IDRI.

Impact of Levofloxacin for the Prophylaxis of Bloodstream Infection on the Gut Microbiome in Patients With Hematologic Malignancy

Background

We evaluated the differential impact of levofloxacin administered for the prophylaxis of bloodstream infections compared with broad-spectrum beta-lactam (BSBL) antibiotics used for the treatment of neutropenic fever on the gut microbiome in patients with hematologic malignancy.

Methods

Stool specimens were collected from patients admitted for chemotherapy or stem cell transplant in the setting of the evaluation of diarrhea from February 2017 until November 2017. Microbiome characteristics were compared among those exposed to levofloxacin prophylaxis vs those who received BSBL antibiotics.

Results

Sixty patients were included, most with acute myeloid leukemia (42%) or multiple myeloma (37%). The gut microbiome of patients with BSBL exposure had significantly reduced Shannon's alpha diversity compared with those without (median [interquartile range {IQR}], 3.28 [1.73 to 3.71] vs 3.73 [3.14 to 4.31]; P = .01). However, those with levofloxacin exposure had increased alpha diversity compared with those without (median [IQR], 3.83 [3.32 to 4.36] vs 3.32 [2.35 to 4.02]; P = .03). Levofloxacin exposure was also associated with a trend toward lower risk of dominance of non-Bacteroidetes genera compared with those without levofloxacin exposure (3 [14%] vs 15 [38%]; P = .051).

Conclusions

The impact of antibiotics on the gut microbiome varies by class, and levofloxacin may disrupt the gut microbiome less than BSBLs in this patient population.

Selection of ESBL-Producing E. coli in a Mouse Intestinal Colonization Model

Asymptomatic human carriage of antimicrobially drug-resistant pathogens prior to infection is increasing worldwide. Further investigation into the role of this fecal reservoir is important for combatting the increasing antimicrobial resistance problems. Additionally, the damage on the intestinal microflora due to antimicrobial treatment is still not fully understood. Animal models are powerful tools to investigate bacterial colonization subsequent to antibiotic treatment. In this chapter we present a mouse-intestinal colonization model designed to investigate how antibiotics select for an ESBL-producing E. coli isolate. The model can be used to study how antibiotics with varying effect on the intestinal flora promote the establishment of the multidrug-resistant E. coli. Colonization is successfully investigated by sampling and culturing stool during the days following administration of antibiotics. Following culturing, a precise identification of the bacterial strain found in mice feces is applied to ensure that the isolate found is in fact identical to the strain used for inoculation. For this purpose random amplified of polymorphic DNA (RAPD) PCR specifically developed for E. coli is applied. This method allows us to distinguish E. coli with more than 99.95% genome similarity using a duplex PCR method.

Fluoroquinolone Prophylaxis Is Highly Effective for the Prevention of Central Line-Associated Bloodstream Infections in Autologous Stem Cell Transplant Patients

Patients undergoing stem cell transplant (SCT) for the treatment of hematologic malignancy are at increased risk for central line-associated bloodstream infections (CLABSIs). The use of prophylactic antibiotics to prevent CLABSIs in the setting of autologous SCT is of unclear benefit. We aimed to evaluate the impact of levofloxacin prophylaxis on reducing CLABSIs in this high-risk population. Patients undergoing autologous SCT at a tertiary care hospital received levofloxacin prophylaxis from January 13, 2016 to January 12, 2017. Levofloxacin was administered from autologous SCT (day 0) until day 13, absolute neutrophil count > 500/mm³, or neutropenic fever, whichever occurred first. Clinical outcomes were compared with a baseline group who underwent autologous SCT but did not receive antibacterial prophylaxis during the previous year. The primary endpoint was incidence of CLABSIs assessed using Cox proportional hazards regression. A total of 324 patients underwent autologous SCT during the entire study period, with 150 receiving levofloxacin prophylaxis during the intervention period. The rate of CLABSIs was reduced from 18.4% during the baseline period to 6.0% during the intervention period. On multivariable analysis levofloxacin prophylaxis significantly reduced CLABSI incidence (hazard ratio, .33; 95% confidence interval [CI], .16 to .69; P = .003). There was also a reduction in the risk of neutropenic fever (odds ratio [OR], .23; 95% CI, .14 to .39; P < .001) and a trend toward a reduction in intensive care unit transfer for sepsis (OR, .33; 95% CI, .09 to 1.24; P = .10) in patients receiving levofloxacin prophylaxis. Notably, there was no increase in Clostridium difficile infection in the levofloxacin group (OR, .66; 95% CI, .29 to 1.49; P = .32). Levofloxacin prophylaxis was effective in reducing CLABSIs and neutropenic fever in patients undergoing autologous SCT. Further studies are needed to identify specific patient groups who will benefit most from antibiotic prophylaxis.

Microbiome mediation of infections in the cancer setting

Infections encountered in the cancer setting may arise from intensive cancer treatments or may result from the cancer itself, leading to risk of infections through immune compromise, disruption of anatomic barriers, and exposure to nosocomial (hospital-acquired) pathogens. Consequently, cancer-related infections are unique and epidemiologically distinct from those in other patient populations and may be particularly challenging for clinicians to treat. There is increasing evidence that the microbiome is a crucial factor in the cancer patient's risk for infectious complications. Frequently encountered pathogens with observed ties to the microbiome include vancomycin-resistant Enterococcus, Enterobacteriaceae, and Clostridium difficile; these organisms can exist in the human body without disease under normal circumstances, but all can arise as infections when the microbiome is disrupted. In the cancer patient, such disruptions may result from interventions such as chemotherapy, broad-spectrum antibiotics, or anatomic alteration through surgery. In this review, we discuss evidence of the significant role of the microbiome in cancer-related infections; how a better understanding of the role of the microbiome can facilitate our understanding of these complications; and how this knowledge might be exploited to improve outcomes in cancer patients and reduce risk of infection.

Discontinuation of Systematic Surveillance and Contact Precautions for Vancomycin-Resistant Enterococcus (VRE) and Its Impact on the Incidence of VRE faecium Bacteremia in Patients with Hematologic Malignancies

OBJECTIVE To study the effect of discontinuation of systematic surveillance for vancomycin-resistant Enterococcus (VRE) and contact isolation of colonized patients on the incidence of VRE bacteremia SETTING A hematology-oncology unit with high prevalence of VRE colonization characterized by predominantly sporadic molecular epidemiology PARTICIPANTS Inpatients with hematologic malignancies and recipients of hematopoietic stem cell transplantation METHODS The incidence of VRE bacteremia was measured prospectively during 2 different 3-year time periods; the first during active VRE surveillance and contact precautions and the second after discontinuation of these policies. We assessed the collateral impact of this policy change on the incidence of bacteremia due to methicillin-resistant Staphylococcus aureus (MRSA) and Clostridium difficile infection even though we maintained contact precautions for these organisms. Incidence of infectious events was measured as number of events per 1,000 patients days per month. Time series analysis was used to evaluate trends. RESULTS The incidence of VRE bacteremia remained stable after discontinuation of VRE surveillance and contact precautions. The incidence of MRSA bacteremia and Clostridium difficile infection for which we continued contact precautions also remained stable. Aggregated antibiotic utilization and nursing hours per patient days were similar between the 2 study periods. CONCLUSION Active surveillance and contact precautions for VRE colonization did not appear to prevent VRE bacteremia in patients with hematologic malignancies and recipients of hematopoietic stem cell transplantation with high prevalence of VRE characterized by predominantly sporadic molecular epidemiology.

Increased Susceptibility to Vancomycin-Resistant Enterococcus Intestinal Colonization Persists After Completion of Anti-Anaerobic Antibiotic Treatment in Mice

Background:

Antibiotic-associated disruption of the indigenous intestinal microflora may persist beyond the treatment period. Although piperacillin/tazobactam inhibits the establishment of vancomycin-resistant Enterococcus (VRE) stool colonization in mice during treatment, we hypothesized that this agent and other anti-anaerobic antibiotics would increase susceptibility to colonization during the period of recovery of the intestinal microflora.

Design:

Mice received 104 colony-forming units of vancomycin-resistant E. faecium by orogastric inoculation 2, 5, or 10 days after completing 5 days of subcutaneous antibiotic treatment, or both during and 2 days after the completion of treatment. Denaturing gradient gel electrophoresis (DGGE) was performed to assess changes in the intestinal microflora.

Results:

Anti-anaerobic antibiotics (ie, piperacillin/tazobactam, cefoxitin, and clindamycin) caused significant disruption of the indigenous microflora (mean DGGE similarity indices ≤ 27% in comparison with saline controls) and promoted the establishment of high-density colonization when VRE was inoculated 2 or 5, but not 10, days following treatment (P < .001). Piperacillin/tazobactam exhibited a biphasic effect on the establishment of colonization (ie, inhibition when exposed to VRE during treatment and promotion when exposed to VRE after discontinuation of treatment), resulting in greater overall promotion of colonization than did agents with minimal anti-anaerobic activity (ie, levofloxacin, cefepime, and aztreonam) when VRE was inoculated both during and 2 days after treatment (P< .001).

Conclusion:

Patients receiving anti-anaerobic antibiotics, including piperacillin/tazobactam, may be susceptible to the establishment of high-density VRE colonization during the period of recovery of the anaerobic microflora.

Antibiotic Exposure and Room Contamination Among Patients Colonized With Vancomycin-Resistant Enterococci

Objective.

To determine whether total and antianaerobic antibiotic exposure increases the risk of room contamination among vancomycin-resistant enterococci (VRE)–colonized patients.

Design And Setting.

A 14-month study in 2 intensive care units at an academic tertiary care hospital in Boston, Massachusetts.

Patients.

All patients who acquired VRE or were VRE-colonized on admission and who had environmental cultures performed.

Methods.

We performed weekly environmental cultures (2 sites per room) and considered a room to be contaminated if there was a VRE-positive environmental culture during the patient's stay. We determined risk factors for room contamination by use of the Cox proportional hazards model.

Results.

Of 142 VRE-colonized patients, 35 (25%) had an associated VRE-positive environmental culture. Patients who contaminated their rooms were more likely to have diarrhea than those who did not contaminate their rooms (23 [66%] of 35 vs 41 [38%] of 107;P= .005) and more likely to have received antibiotics while VRE colonized (33 [94%] of 35 vs 86 [80%] of 107;P= .02). There was no significant difference in room contamination rates between patients exposed to antianaerobic regimens and patients exposed to nonantianaerobic regimens or between patients with and patients without diarrhea, but patients without any antibiotic exposure were unlikely to contaminate their rooms. Diarrhea and antibiotic use were strongly confounded; although two-thirds of room contamination occurred in rooms of patients with diarrhea, nearly all of these patients received antibiotics. In multivariable analysis, higher mean colonization pressure in the ICU increased the risk of room contamination (adjusted hazard ratio per 10% increase, 1.44 [95% confidence interval, 1.04–2.04]), whereas no antibiotic use during VRE colonization was protective (adjusted hazard ratio, 0.21 [95% confidence interval, 0.05–0.89]).

Conclusions.

Room contamination with VRE was associated with increased mean colonization pressure in the ICU and diarrhea in the VRE-colonized patient, whereas no use of any antibiotics during VRE colonization was protective.

Vancomycin resistant enterococci: from the hospital effluent to the urban wastewater treatment plant

Vancomycin is an important antibiotic to treat serious nosocomial enterococci infections. Human activities, in particular those related with clinical practices performed in hospitals, can potentiate the transfer and selection of clinically-relevant resistant bacteria such as vancomycin resistant enterococci (VRE). Indeed, previous studies demonstrated the occurrence of VRE in urban wastewater treatment plants and related environments (e.g. sewage, rivers). In this study, the occurrence of VRE in a hospital effluent and in the receiving urban wastewater treatment plant was investigated. Vancomycin and ciprofloxacin resistant bacteria occurred in the hospital effluent and in raw municipal inflow at densities of 10(3) to 10(2) CFU mL(-1), being significantly more prevalent in the hospital effluent than in the urban wastewater. Most of the VRE isolated from the hospital effluent belonged to the species Enterococcus faecalis and Enterococcus faecium and presented multidrug-resistance phenotypes to ciprofloxacin, tetracycline, erythromycin, and high-level gentamicin. The same pattern was observed in clinical isolates and in enterococci isolated from the final effluent of the urban wastewater treatment plant. These results show that hospital effluents discharged into urban wastewater treatment plants may be a relevant source of resistance spread to the environment.

High-level ciprofloxacin resistance among hospital-adapted Enterococcus faecium (CC17)

Hospital-adapted Enterococcus faecium differ from their colonising variants in humans and animals by additional genomic content. Molecular typing based on multilocus sequence typing (MLST) allows allocation of isolates to specific clonal complexes (CCs), such as CC17 for hospital-adapted strains. Acquired ampicillin resistance is a specific feature of these hospital isolates, especially in Europe. A few recent reports have described acquired high-level ciprofloxacin resistance as a supposed feature of hospital-adapted E. faecium strains. In the present retrospective analysis, ciprofloxacin minimum inhibitory concentrations (MICs) of 609 clinical isolates from German hospital patients (1997-2007) were determined and a breakpoint for high-level resistance was deduced (>16mg/L). Acquired high-level ciprofloxacin resistance was distributed among isolates of 26 different MLST types (all CC17), indicating a wide prevalence of this acquired resistance trait among the hospital-adapted E. faecium population. High-level ciprofloxacin resistance was linked to gyrA and parC mutations in 98 investigated isolates. Eleven different allele types or combinations thereof were identified. Their allocation to specific MLST and pulsed-field gel electrophoresis (PFGE) types revealed differences in the emergence and spread of corresponding mutations and strains.

A mouse model for characterization of gastrointestinal colonization rates among environmental Aeromonas isolates

The colonization rates of 10 different environmental Aeromonas isolates were determined using a novel mouse-streptomycin pretreatment method. As demonstrated, alterations to the colon flora of mice pretreated with streptomycin allowed transient colonization by bacterial species normally excluded by host competition. A novel procedure is described for determining the colonization abilities of Aeromonas isolates under these conditions. The colonization rates of A. salmonicida, A. encheleia, and A. allosaccharophila were either negative or occurred randomly at low levels with respect to concentrations of the dosage consumed by the animals. In contrast, A. hydrophila, A. veronii biovar sobria, and A. caviae exhibited relatively high rates of mouse colon tissue colonization.

Pharmacokinetics/Pharmacodynamics for critical care clinicians

For appropriate antibiotic therapy and selection, the clinician must be familiar with pharmacodynamic concepts that integrate an antibiotic's microbiologic activity, pharmacokinetic properties, and mode of bacterial killing. Much of the traditional dosing methods that continue to this day are based more on habit rather than science. This article addresses these issues and explains the basis for the new scientific ways to administer antibiotics to optimize patient outcomes.

The categorization of prior antibiotic use: impact on the identification of risk factors for drug resistance in case control studies

BACKGROUND

Analytic approaches to the identification of risk factors for the development of drug resistance vary and may affect study results. Using fluroquinolone-resistant Pseudomonas aeruginosa (FQRPA) and imipenem-resistant P. aeruginosa as models (IRPA), we aimed to examine the effect of different approaches to classification of prior antibiotic use (i.e., class versus spectrum) on the identification of risk factors for antibiotic resistance.

METHODS

Case-control studies to identify risk factors for FQRPA and IRPA were performed. In each, two analytic models were utilized. In the first, prior antibiotic use was classified by class, and in the other, prior antibiotic use was classified by spectrum of activity. Risk factors identified by the two models were compared qualitatively for each resistant organism.

RESULTS

879 isolates of P. aeruginosa were included in the case-control studies. Risk factors for FQRPA and IRPA identified in multivariable analyses differed based on which method of classification of prior antibiotic use was utilized.

CONCLUSIONS

The identification risk factors for the development of drug-resistant organisms could depend on the method of classification of prior antibiotic use. In studies of risk factors for resistant infections, the approach to classification of prior antibiotic use should be clearly stated and justified.

Investigating the association between antibiotic use and antibiotic resistance: impact of different methods of categorising prior antibiotic use

Many studies have explored the association between antibiotic use and antibiotic resistance. However, methods employed in these studies to categorise prior antibiotic use (e.g. by class, by spectrum) have not been well described. The impact of using different categorisation methods on identifying risk factors for resistance is unknown. To explore these issues, we focused on extended-spectrum beta-lactamase-producing Escherichia coli and Klebsiella spp. (ESBL-EK) as a model. First, we conducted a systematic review of studies of risk factors for ESBL-EK to characterise past approaches to categorising antibiotic use. Second, we re-analysed data from a prior study of risk factors for ESBL-EK. Two separate multivariate models of risk factors for ESBL-EK were constructed: one with prior antibiotic use categorised by class and the other with prior antibiotic use categorised by spectrum of activity. Among the 20 articles that met the inclusion criteria for the systematic review, there was tremendous variability in how prior antibiotic use was categorised (e.g. by agent, class, spectrum and/or a combination of these). No study justified its choice of categorisation method. In the re-analysis of the existing data set, multivariate models of risk factors for ESBL-EK using 'class' and 'spectrum' categorisations differed substantially. In conclusion, there has been no consistent approach to categorising antibiotic use in studies of risk factors for ESBL-EK. Different categorisation schemes were shown to have a substantial impact on study results, particularly for the antibiotic exposures associated with resistance. Elucidating these issues is critical if effective strategies to curb resistance are to be designed.

Fluoroquinolones and Anaerobes

The usefulness of fluoroquinolones for the treatment of mixed aerobic and anaerobic infections has been investigated since these agents started being used in clinical practice. Newer compounds have increased in vitro activity against anaerobes, but clinically relevant susceptibility breakpoints for these bacteria have not been established. Pharmacodynamic analyses and corroboration by new data from clinical trials have enhanced our knowledge concerning the use of fluoroquinolones to treat selective anaerobic pathogens. These studies suggest that newer agents could be useful in the treatment of several types of mixed aerobic and anaerobic infections, including skin and soft-tissue, intra-abdominal, and respiratory infections. The major concerns with expanding the use of fluoroquinolones to treat anaerobic infections have been reports of increasing resistance in Bacteroides group isolates and the impact of these antibiotics on the incidence of Clostridium difficile-associated disease.

Spread of ampicillin/vancomycin-resistant Enterococcus faecium of the epidemic-virulent clonal complex-17 carrying the genes esp and hyl in German hospitals

The incidence of vancomycin-resistant Enterococcus faecium isolation was low (<or=5%) in German hospitals before 2003. Within the second half of 2003 and the first half of 2004, however, increasing frequencies of up to 14% were noticed in several hospitals in southwestern Germany. This increase was attributed mainly to the occurrence and spread of epidemic-virulent ampicillin/vancomycin-resistant, vanA- and vanB-positive E. faecium clones, most of which exhibited the virulence factors enterococcal surface protein (esp) and bacteriocin activity and some which exhibited hyaluronidase (hyl). E. faecium possessing hyaluronidase was initially found in U.S. hospitals and recently detected in several European hospitals and, subsequently, in German hospitals as well. Ampicillin/vancomycin-resistant E. faecium clones originating mainly from southwestern German hospitals were characterized by multilocus sequence typing since different sequence types (STs) belonging to the clonal complex-17 are currently disseminated worldwide. Multilocus sequence typing revealed that, in 1998 and 1999, ampicillin/vancomycin-resistant E. faecium clone ST-117 was prevalent in various German hospitals, while in 2003 and 2004, clone ST-203 dominated in several hospitals located in southwestern Germany. Both sequence types display single-locus variants of ST-78, which was frequently recorded in various Italian hospitals between 2000 and 2003, and all of these STs belong to the clonal complex-17. Expression of linezolid resistance was observed in ampicillin/glycopeptide-resistant E. faecium strains (VanA type) from two tertiary hospitals in southwestern Germany due to mutations in domain V of the 23S rDNA (G2576T). While in one hospital the resistance emerged during linezolid therapy, in the other hospital resistance was caused by transfer of an identical linezolid/ampicillin/glycopeptide-resistant E. faecium strain. In conclusion, it is very important to monitor the occurrence of epidemic-virulent clonal complex-17 strains of E. faecium to prevent their spread in hospitals, especially if they are resistant to glycopeptides and linezolid.

Antibiotics and gastrointestinal colonization by vancomycin-resistant enterococci

Although several classes of antimicrobial agents have been associated with colonization or infection with glycopeptide-resistant enterococci (GRE) in individual clinical studies, the agents most commonly implicated are extended-spectrum cephalosporins and compounds with potent activity against anaerobic bacteria, including ticarcillin-clavulanic acid. In some clinical studies, formulary alterations designed to minimize the use of extended-spectrum cephalosporins or ticarcillin-clavulanic acid have resulted in significant decreases in colonization and infection by GRE. Experimental data using a mouse model of GRE gastrointestinal colonization indicate that persistence of high-level GRE colonization of the mouse gastrointestinal tract is promoted by exposure to agents with potent activity against anaerobic bacteria, suggesting that reduction of competing flora is the major factor leading to persistence of high-level colonization. One study performed in humans is consistent with this model and suggests that high levels of colonization may promote spread of resistant organisms in the nosocomial setting. Establishing colonization with GRE in uncolonized mice correlates with exposure to agents that are (a) secreted into the bile in significant concentrations and (b) have negligible activity against the colonizing enterococcal strain. Differences between piperacillin-tazobactam and ceftriaxone in the establishment model can be attributed directly to differences in their anti-enterococcal activity. Modification of antimicrobial prescribing practices may play an important role in facilitating successful infection control efforts to limit GRE in the nosocomial setting.

Effect of antibiotic treatment on growth of and toxin production by Clostridium difficile in the cecal contents of mice

In mice, subcutaneous administration of antibiotics that disrupt the anaerobic microflora (i.e., clindamycin, piperacillin-tazobactam, and ceftriaxone) facilitated in vitro growth of and toxin production by Clostridium difficile in cecal contents, whereas antibiotics that cause minimal disruption of the anaerobic microflora (i.e., levofloxacin, cefepime, and aztreonam) did not.