Impact of Severity of Illness Bias and Control Group Misclassification Bias in Case-Control Studies of Antimicrobial-Resistant Organisms

Background:

Case-control studies often analyze risk factors for antibiotic resistance. Recently published articles have illustrated that randomly selected control-patients may be preferable to those with the susceptible phenotype of the organism. A possible methodologic problem with randomly selected control-patients is potential bias due to control group misclassification. This occurs if some control-patients did not have clinical cultures performed and thus might have been unidentified case-patients. If this bias exists, these studies might be expected to report lower odds ratios (ORs) because control-patients would be more like case-patients.

Objective:

To analyze potential biases that might arise due to control group misclassification and potentially larger selection biases that may be introduced if control-patients are required to have at least one clinical culture.

Patients:

One hundred twenty case-patients, 770 control-patients in group 1, and 510 control-patients in group 2.

Methods:

Two case-control studies. Case-patients had clinical cultures positive for imipenem-resistant Pseudomonas aeruginosa. The first group of control-patients were random. The second group of control-patients were identical to those in group 1 except being required to have at least one clinical culture.

Results:

Univariate analyses showed higher ORs for case-patients versus control-patients in group 1 (imipenem [OR, 12.5], piperacillin-tazobactam [OR, 3.7], and vancomycin [OR, 4.7]) as compared with case-patients versus control-patients in group 2 (imipenem [OR, 8.0], piperacillin-tazobactam [OR, 2.5], and vancomycin [OR, 3.0]).

Conclusion:

Requiring control-patients to have at least one clinical culture introduces a selection bias likely because it eliminates patients with less severe illness.

Fatal Pseudomonas aeruginosa pneumonia in a previously healthy woman was most likely associated with a contaminated hot tub

Community-acquired pneumonia due to Pseudomonas aeruginosa in previously healthy individuals is a rare disease that is associated with high fatality. On 14 February 2010 a previously healthy 49-year-old woman presented to an emergency room with signs and symptoms of pneumonia, 2 days after returning from a spa holiday in a wellness hotel. Blood cultures and respiratory specimens grew P. aeruginosa. Despite adequate antimicrobial therapy, the patient died of septic multiorgan failure on day nine of hospitalization. On February 26, nine water samples were taken from the hotel facilities used by the patient: In the hot tub sample 37,000 colony-forming units of P. aeruginosa/100 ml were detected. Two of five individual colonies from the primary plate used for this hot tub water sample were found to be genetically closely related to the patients’ isolates. Results from PFGE, AFLP and MLST analysis allowed the two lung isolates gained at autopsy and the whirlpool bathtub isolates to be allocated into one cluster. The patient most likely acquired P. aeruginosa from the contaminated water in the hotel’s hot tub. The detection of P. aeruginosa in high numbers in a hot tub indicates massive biofilm formation in the bath circulation and severe deficiencies in hygienic maintenance. The increasing popularity of hot tubs in hotels and private homes demands increased awareness about potential health risks associated with deficient hygienic maintenance.

Do New Zealand children with non-cystic fibrosis bronchiectasis show disease progression?

BACKGROUND

There is minimal literature available on the long-term outcome of pediatric non-cystic fibrosis (CF) bronchiectasis.

AIM

To document 5-year outcomes of children with chest computerized tomography (CT) scan diagnosed bronchiectasis from a tertiary New Zealand (NZ) respiratory clinic.

METHODS

Review of a clinical database identified 91 children. Demographics, clinical data, lung function, chest X-ray (CXR), sputum, presumed etiology, admission data, and the NZ deprivation index (NZDep) were collected. Univariate and multivariate regression were used to correlate clinical findings with lung function data and CXR scores using the Brasfield Scoring System.

RESULTS

Of the 91 children, 53 (59%) were Pacific Island, 22 (24%) Maori, 14 (15%) European, and 2 (2%) Other. The median follow-up period was 6.7 years (range 5.0-15.3 years) and median age at diagnosis was 7.3 years (range 11 months-16 years). Lung function data (n = 64) showed a mean decline of -1.6% predicted/year. In 30 children lung function declined (mean FEV(1) -4.4% predicted/year, range 1-17%), remained stable in 13 and improved in 21 children (mean FEV(1) of +3% predicted/year, range 1-15%). Reduced lung function was associated with male gender, chronic Haemophilus influenzae infection, longevity of disease, and Maori and Pacific Island ethnicity. There was a significant correlation with FEV(1) and CXR score at beginning (n = 47, r = 0.45, P = 0.001) and end (n = 26, r = 0.59, P = 0.002) of the follow-up period. The only variable consistently related to CXR score was chronic Haemophilus influenzae infection occurring in 27 (30%) (r(2)  = 0.52, P = <0.0001). Only four children were chronically infected with Pseudomonas species. Six children died.

CONCLUSION

In our experience despite management in a tertiary multidisciplinary bronchiectasis clinic, progression of lung disease continues in a group of children and young adults.

DNA microarray for genotyping multidrug-resistant Pseudomonas aeruginosa clinical isolates

The management of infections with multidrug-resistant Pseudomonas aeruginosa needs fast and reliable methods of antibiotic susceptibility testing for a therapy improvement. For this purpose, we developed a DNA microarray for genotyping antibiotic resistance and a few virulence factors. The array covers mutations in the efflux regulators mexR, nfxB, mexT, gyrase gyrA, and parC, as well as plasmid-encoded vim, imp, oxa, aph, aac, and aad genes, and virulence-associated mucA and exoU, exoT, and exoS genes, respectively. The whole procedure can be performed in less than 5 h and consists of DNA isolation, target gene amplification, fluorescence labeling, fragmentation, and array hybridization. Concerning the genotype-phenotype comparison in the test collection, the coverage of relevant resistance determinants for antibiotics used in a calculated therapy of critical ill patients was 87.8%.

Transmission of Pseudomonas aeruginosa in children with cystic fibrosis attending summer camps in The Netherlands

BACKGROUND

Cross-infection of Pseudomonas aeruginosa has been reported to occur at holiday camps for children with Cystic Fibrosis (CF) with varying frequency. The study aimed to establish the degree of transmission resulting in subsequent infection of P. aeruginosa among CF children (n=80) attending holiday camps in The Netherlands.

METHODS

The study was performed in the summer of 2001 in four camps organised simultaneously at different locations. Sputum was collected on day 1 of the holiday, and three and six months later. Different morphotypes of P. aeruginosa from sputum were genotyped by AFLP analysis. Criteria were defined for the degree of evidence of transmission.

RESULTS

There were 18 cases possible, 2 cases of probable transmission and 1 case of highly probable transmission. Two predominant types of P. aeruginosa were found (types 18 and 23). Type 18 was already prevalent on day 1 mostly in younger children and was involved in eleven cases of transmission; type 23 was involved in six cases of transmission among older children.

CONCLUSIONS

There was a considerable risk of transmission of P. aeruginosa during holiday camps for CF children in The Netherlands. Two genotypes of P. aeruginosa appeared to be easily transmissible, one of which seemed common in the Dutch CF population.

Comparison of antimicrobial cycling and mixing strategies in two medical intensive care units*

OBJECTIVE

To compare a mixing vs. a cycling strategy of use of anti-Pseudomonas antibiotics on the acquisition of resistant Gram-negative bacilli in the critical care setting.

DESIGN

Prospective, open, comparative study of two strategies of antibiotic use.

SETTING

Two medical intensive care units of a university hospital.

PATIENTS

A total of 346 patients admitted for >or=48 hrs to two separate medical intensive care units during an 8-month period.

INTERVENTIONS

Patients, who according to the attending physician's judgment required an anti-Pseudomonas regimen, were assigned to receive cefepime/ceftazidime, ciprofloxacin, a carbapemen, or piperacillin-tazobactam in this order. "Cycling" was accomplished by prescribing one of these antibiotics during 1 month each. "Mixing" was accomplished by using the same order of antibiotic administration on consecutive patients. Interventions were carried out during two successive 4-month periods, starting with mixing in one unit and cycling in the other.

MEASUREMENTS AND MAIN RESULTS

Swabbing of nares, pharynx, and rectum and culture of respiratory secretions were obtained thrice weekly. The main outcome variable was the proportion of patients acquiring enteric or nonfermentative Gram-negative bacilli resistant to the antibiotics under intervention. The scheduled cycling of antibiotics was only partially successful. Although the expected antibiotic was the most prevalent anti-Pseudomonas agent used within the corresponding period, it never accounted for >45% of all anti-Pseudomonas antimicrobials administered. During mixing, a significantly higher proportion of patients acquired a strain of Pseudomonas aeruginosa resistant to cefepime (9% vs. 3%, p = .01), and there was a trend toward a more frequent acquisition of resistance to ceftazidime (p = .06), imipenem (p = .06), and meropenem (p = .07). No differences in the rate of acquisition of potentially resistant Gram-negative bacilli or incidence of intensive care unit-acquired infections and infections due to particular organisms were observed.

CONCLUSIONS

In critically ill medical patients, a strategy of monthly rotation of anti-Pseudomonas beta-lactams and ciprofloxacin may perform better than a strategy of mixing in the acquisition of P. aeruginosa resistant to selected beta-lactams.

Phenotypic and genotypic characteristics of Pseudomonas aeruginosa strains isolated from hospitals in the north-west region of Poland

A total of 90 Pseudomonas aeruginosa strains isolated from 4 hospitals in the west-north region of Poland were studied by arbitrarily primed polymerase chain reaction (AP-PCR). AP-PCR results revealed the presence of 11 main groups of patterns (A-K) and 5 unique patterns among isolates. Generally, they were characterized by high resistance to antibiotics tested and significant differences in serogroups and types of growth on Cetrimide Agar medium. It was observed that clonally related strains were isolated from patients within the same ward, among different wards as well as in distant hospitals.

Characterization of Pseudomonas aeruginosa isolated from chronically infected children with cystic fibrosis in India

BACKGROUND

Pseudomonas aeruginosa is the leading cause of morbidity and mortality in patients with cystic fibrosis (CF). With chronicity of infection, the organism resides as a biofilm, shows multi-drug resistance, diversifies its colony morphology and becomes auxotrophic. The patients have been found to be colonized with multiple genotypes. The present work was carried out to characterize P. aeruginosa isolated from children with cystic fibrosis using phenotypic and genotypic methods.

RESULTS

We studied 56 patients with CF attending the Pediatric Chest clinic at All India Institute of Medical Sciences, New Delhi, India during August 1998-August 2001. These patients were regularly followed up at the clinic. Out of 56 patients, 27 were culture positive for P. aeruginosa where 8 were chronically infected (Group1) and 19 were intermittently colonized with the organism (Group2). Patients under Group1 had significantly higher rates of hospitalization, death and colonization with different colony morphotypes (p < 0.05). The isolates from Group1 patients were the positive producers of extended spectrum beta lactamase. A total of 5 auxotrophs were recovered from 2 patients where one was chronically infected with P. aeruginosa and the other was a recently enrolled patient. The auxotrophs had the specific requirement for methionine and arginine. Molecular typing revealed 33 ERIC-PCR (E1-E33) and 5 PCR-ribotyping (P1-P5) patterns. By ERIC-PCR, 4 patients were colonized with 2-4 genotypes and the remaining 23 patients were colonized with the single genotype.

CONCLUSION

With chronicity of infection, P. aeruginosa becomes multidrug resistant, diversifies its colony morphology, acquires mucoidity and shows auxotrophy for amino acids. The chronically infected patients can be colonized with multiple genotypes. Thus in a particular clinical set up, high index of suspicion should be there for diagnosis of CF patients so as to prevent the delay in diagnosis and management of CF patients.

Long-Term Results of Multiple-Stage Treatment for Posttraumatic Osteomyelitis of the Tibia

BACKGROUND

The treatment of posttraumatic osteomyelitis of the tibia requires meticulous debridement and adequate soft tissue coverage. At our institution, we perform a staged procedure consisting of surgical debridement followed by muscle coverage. If necessary, implantation of a cancellous iliac bone graft was always performed as a three-stage treatment.

METHODS

We performed a retrospective analysis of 47 patients treated for posttraumatic osteomyelitis of the tibia between 1987 and 1998.

RESULTS

Twenty-two patients originally had a Gustilo grade III fracture, 21 patients had a Gustilo grade I or II or closed fracture, the Gustilo grade was not known for 2 patients, and 2 patients had no fracture. Using the Cierny-Mader classification, most patients had a localized osteomyelitis. To cover the debrided area, 20 pedicled muscle transfers and 28 microvascular free flaps were used; one patient had two localizations of osteomyelitis (both proximal and distal) and received two muscle flaps. Flap failure was 8% and was successfully treated by additional flap coverage in two cases; one was closed by a split skin graft and one was closed by secundum. Twenty-six patients received a cancellous bone graft. During an average follow-up of 94 months, 9% had a recurrence of osteomyelitis for which additional surgical interventions were necessary. Finally, all the infections were eventually cured.

CONCLUSION

Our staged surgery proved to be an excellent method of treating osteomyelitis after open or closed fractures of the tibia.

Quelles stratégies découlent de la surveillance des bactéries multirésistantes aux antibiotiques afin de mieux maîtriser leur diffusion ?

Should we screen for colonization to control the spread of multidrug-resistant bacteria? A multidrug-resistant bacteria surveillance program was performed in 1999 at Laënnec Hospital (Nantes, France). After a 3-year period, the results permit us to determine the strategy to strengthen their spread. In 2001, Staphylococcus aureus resistant to methicillin represented 45% of the 202 multidrug-resistant bacteria isolated. The global incidence rate per 100 admissions remained stable between 1999 and 2001 (0.42%), but those of infections acquired in our institution decreased significantly from 0.27% in 1999 to 0.18% in 2001 (P < 0.05), particularly in medical care units (P < 0.04). In spite of this surveillance program and hygiene trainings, the global incidence remained stable during the study period, even if our action contributed to decrease the incidence of S. aureus resistant to methicillin acquired in our institution. Isolation precautions and screening for colonization policy in intensive care units are not sufficient to control the spread of MRB at hospital level. They should be strengthened by procedures for the transfer of infected or colonized patients and by antibiotic use control.

Evaluation of a new definition for chronic Pseudomonas aeruginosa infection in cystic fibrosis patients

BACKGROUND

Patients were defined each successive month as either 'chronic' when more than 50% of the preceding 12 months were PA culture positive, 'intermittent' when < or =50% of the preceding 12 months were PA culture positive, 'free of PA', with no growth of PA for the previous 12 months, having previously been PA culture positive, or 'never infected', when PA had never been cultured.

METHODS

Cross-sectional analysis of 146 children attending the Leeds Regional Cystic Fibrosis Centre was performed to assess relationship between the new definition and clinical scores and investigations. The response variable was regressed on age and sex and the residuals analysed using the Kruskal-Wallis test.

RESULTS

The 'chronic' group (18% of patients) had significantly worse Shwachman-Kulczycki (SK) and Northern chest X-ray scores, and % predicted FEV(1) values than the 'free' (28%) or 'never' (20%) categories (P<0.004). The 'intermittent' group (34%) had a significantly higher SK score than the 'chronic' group (P<0.0001), and a significantly lower % predicted FEV(1) value than the 'free' or 'never' groups (P<0.0003). 'Chronic' patients were significantly associated with a positive, and 'never' patients with a negative, PA antibody result (P<0.001).

CONCLUSIONS

The validity and importance of identifying these four subgroups is demonstrated. Previous definitions may over-estimate the prevalence of chronic infection.

Nosocomial infections caused by multiresistant Pseudomonas aeruginosa

A case-control study was done to evaluate factors associated with nosocomial infections by multiresistant Pseudomonas aeruginosa (MRPA). Results showed that MRPA was associated with the use of immunosuppressive and antimicrobial drugs. Five typing methods indicated that the MRPA infections were due to multiple strains rather than a single strain.

Pseudomonas aeruginosa outer-membrane protein F epitopes are highly immunogenic in mice when expressed on a plant virus

A synthetic peptide (peptide 10) representing a surface-exposed, linear B cell epitope from outer-membrane (OM) protein F of Pseudomonas aeruginosa was shown previously to afford protection in mice from P. aeruginosa infection. This peptide was expressed in tandem with the protein F peptide 18 on each of the two coat proteins of cowpea mosaic virus (CPMV). The chimaeric virus particles (CVPs) expressing the peptides on the S (small) coat protein (CPMV-PAE4) and L (large) coat protein (CPMV-PAE5) were used to immunize mice. Following subcutaneous immunization in Freund's and QuilA adjuvants, CPMV-PAE4 induced antibodies predominantly against peptide 18, whereas CPMV-PAE5 produced antibodies exclusively against peptide 10, indicating that the site of peptide expression on CPMV influences its immune recognition. The anti-peptide antibodies elicited by CPMV-PAE5 were predominantly of the IgG2a isotype, indicating a highly polarized TH1-type response. The peptide-specific IgG2a strongly recognized the whole F protein, but more importantly, recognized protein F in all seven Fisher-Devlin immunotypes of P. aeruginosa. Furthermore, the peptide-specific IgG2a in CVP/QS-21 adjuvant-immunized mice was shown to bind complement and to augment phagocytosis of P. aeruginosa by human neutrophils in vitro. The ability of CPMV-PAE5 to induce P. aeruginosa-specific opsonic IgG2a gives it potential for further development as a protective vaccine against P. aeruginosa.

Phenotypic heterogeneity of Pseudomonas syringae van Hall

The study of phenotypic properties of 108 strains of Pseudomonas syringae pv. syringae van Hall isolated from Cherry laurel (50 strains) and various host plants (58 strains) and 53 strains of other pathovars of P. syringae and fluorescent Pseudomonas showed that the majority of the strains (91/108) were clustered in one phenon (phenon 14) containing strains commonly considered as P.s. pv. syringae. The present type strain of P.s. pv. syringae was distantly related to phenon 14. Other pathovars of P. syringae constituted 13 discrete phenons.

[Classification of acute bacterial enteropathies]

The first part considers pathogenic microorganisms (Vibrio cholerae and parahaemolytic vibrio, Clostridium welchii, enteropathogenic E. coli, Shigella, Salmonella, other enterobacteria and pseudomonas. Yersinia, simply enterotoxic Staphylococcus and that producing acute enteritis) and the process of infection (formation of a surface link without endocellular penetration with elaboration of hexotoxins, formation of a surface link with subsequent intracellular penetration, submucosa penetration). The second part discusses Salmonellae on the basis of personal experience. Particular attention is paid to current aspects of Salmonella microbiological pathomorphosis, the various isolated serotypes in relation to carriers or patients, biochemical atypias of Salmonellae strains, present-day aspects of resistance to chemoantibiotic treatment and the transfer of Salmonella Wien resistances.