The changing epidemiology of severe infections in the ICU

Rapid killing of Acinetobacter baumannii by polymyxins is mediated by a hydroxyl radical death pathway

Acinetobacter baumannii is an opportunistic pathogen that is a cause of clinically significant nosocomial infections. Increasingly, clinical isolates of A. baumannii are extensively resistant to numerous antibiotics, and the use of polymyxin antibiotics against these infections is often the final treatment option. Historically, the polymyxins have been thought to kill bacteria through membrane lysis. Here, we present an alternative mechanism based on data demonstrating that polymyxins induce rapid cell death through hydroxyl radical production. Supporting this notion, we found that inhibition of radical production delays the ability of polymyxins to kill A. baumannii. Notably, we demonstrate that this mechanism of killing occurs in multidrug-resistant clinical isolates of A. baumannii and that this response is not induced in a polymyxin-resistant isolate. This study is the first to demonstrate that polymyxins induce rapid killing of A. baumannii and other Gram-negatives through hydroxyl radical production. This significantly augments our understanding of the mechanism of polymyxin action, which is critical knowledge toward the development of adjunctive therapies, particularly given the increasing necessity for treatment with these antibiotics in the clinical setting.

The increasing role of Acinetobacter species as nosocomial pathogens

Among gram-negative organisms playing a significant role in nosocomial infections, Acinetobacter species have attracted increasing attention in intensive care units during the past two decades. Acinetobacter species are implicated in a wide spectrum of infections (eg, bacteremia, nosocomial pneumonia, urinary tract infections, secondary meningitis, superinfections in burn patients). One of the most striking features of Acinetobacter species is their extraordinary ability to develop multiple resistance mechanisms against major antibiotic classes. They have become resistant to broad-spectrum β-lactams (third-generation cephalosporins, carboxypenicillins, and increasingly to carbapenems); they produce a wide range of aminoglycoside-inactivating enzymes; and most strains are resistant to fluoroquinolones. In Acinetobacter nosocomial infections, the major problems confronting clinicians in intensive care units are related to the severity of Acinetobacter nosocomial infections and to resistance to major antibiotic classes of these organisms.

Timing and aetiology of bacterial infections in a liver intensive care unit

We undertook a prospective study of 887 consecutive adult patients admitted over an 11 year period to a liver intensive care unit. One or more bacterial infections occurred in 335 (37.8%) patients. Gram-positive cocci predominated. In relation to the date of admission these infections occurred in a statistically significant sequence. Streptococci infections were earliest (median time to infection two days), followed by Staphylococcus aureus (three days), coagulase-negative staphylococci (six days) and enterococci (eight days). Escherichia coli infections occurred earlier than those due to klebsiella-enterobacter (two vs seven days; P = 0.0001) and, overall, Enterobacteriaceae earlier than non-fermentative Gram-negatives (four vs. eight days; P = 0.0081). This study contributes to the management of high-dependency patients by confirming statistically the timing and sequence of infecting bacteria in patients with acute liver failure.

Hospital-acquired infections following the 1999 Marmara earthquake

In this study, medical records of all casualties admitted to our hospital following the Marmara earthquake, which struck northwest Turkey and resulted in the destruction of several towns in the Marmara region, were evaluated retrospectively. The time buried under the rubble, demographic data, type of medical and surgical therapies performed, type of injury and data on infection were analysed. Between 17 August and 25 September 1999, 630 trauma victims were received at our hospital and 532 (84%) of them were hospitalized. The mean age of hospitalized patients (312 males, 220 females) was 32 years (2-90 years). Two hundred and twenty patients were hospitalized for more than 48 h. Forty-one of them (18.6%) had 43 hospital-acquired infection (HAI) episodes, which were mostly wound infections (46.5%). A total of 143 culture specimens was collected and 48 yielded the following potential pathogens: 15 Acinetobacter baumanii (31.2%), nine Staphylococcus aureus (18.7%), seven Pseudomonas aeruginosa (14.6%), six Escherichia coli (12.5%), six Klebsiella pneumoniae (12.5%), two Stenotrophomonas maltophilia (4.2%) and three various Pseudomonas spp. (6.3%). All S. aureus strains were found to be resistant to methicillin in vitro. Two strains of A. baumannii and one P. aeruginosa were found to be resistant to all antimicrobials including carbapenems. Fifty-three victims died (10%) and 36 of those died during the first 48 h because of severe injuries and multi-organ failure. After 48 h of hospitalization, the mortality rate was significantly higher in those patients with HAI (14/41) than those without (3/179) (34.1% vs. 1.7%, P<0.05). In conclusion, trauma is the significant factor associated with HAI and a high incidence of Acinetobacter strains was responsible for HAI in trauma patients.

Antibacterial activity of 41 antimicrobials tested against over 2773 bacterial isolates from hospitalized patients with pneumonia: I--results from the SENTRY Antimicrobial Surveillance Program (North America, 1998)

Pneumonia is the second most frequent cause of nosocomial infection, and hospitalization frequently is needed for community-acquired pneumonia. Knowledge of causative pathogens through periodic surveillance, and their prevailing antimicrobial susceptibility patterns becomes paramount in choosing appropriate empiric therapy. The SENTRY Antimicrobial Surveillance Program, tracks pathogen distribution worldwide since 1997 and documents emerging resistance to a wide range of antimicrobial agents. During the respiratory disease season in 1998, each of 30 medical centers (25 in the United States [US], and five in Canada [CAN]) contributed 100 consecutive isolates obtained from hospitalized patients with suspected pneumonia. The 2773 organisms, processed by the monitor consisted of a total of 35 species, with Staphylococcus aureus comprising 25.6% of all isolates and five other species (Pseudomonas aeruginosa 18.7%, Haemophilus influenzae 9.4%, Streptococcus pneumoniae 7.8%, Klebsiella spp. 7.0%, and Enterobacter spp. 6.7%) making up almost 50% of the total. In the US, pneumococci (8.5%) were more prevalent than in CAN (4.1%; p = 0.001). The US isolates of S. pneumoniae were variably susceptible to penicillin (76.8%), with non-susceptible strains demonstrating greater levels of cross resistance to macrolides (31.8%), cefepime (9.0%) and cefotaxime (6.8%), but remaining susceptible to gatifloxacin and quinupristin/dalfopristin. H. influenzae and Moraxella catarrhalis were generally ampicillin-resistant, 40.4-44.4% and 93.7-95.7%, respectively. P. aeruginosa remained very susceptible to amikacin (91.3-93.8%) > tobramycin > meropenem > piperacillin/tazobactam > gentamicin > piperacillin > cefepime (80.0-81.8%). Extended spectrum beta-lactamase phenotypes among the Klebsiella spp. were isolated from five medical centers in the US and were 4.8-6.0% overall; a rate similar to the previous year. Among the US isolates of Enterobacter spp., only 77.6% and 79.6% were susceptible to ceftazidime and cefotaxime, respectively, but >90% were inhibited by cefepime, imipenem, meropenem, aminoglycosides, and fluoroquinolones. Isolates from CAN were generally more susceptible, except for Pseudomonas isolates, where resistance to aminoglycosides, fluoroquinolones and imipenem was greater. The SENTRY Program results outline important national differences in the frequencies of pathogen occurrence, but more importantly, identify unstable patterns of resistance to available antimicrobial drugs, and serves as a reference for results of other local, national or international investigations.

Bacterial agents of lower respiratory tract infections (LRTIs), beta-lactamase production, and resistance to antibiotics in elderly people. DEDALO Study Group

This study determined the etiology of lower respiratory tract infections in the elderly and assessed whether the growth of beta-lactamase producing bacteria is particularly favoured in these patients. Between December 1998 and May 1999, 187 patients with community-acquired pneumonia (CAP), and 887 patients with acute exacerbations of chronic obstructive pulmonary disease (AECOPD) were enrolled. The mean age was 74 years (range of 65-94 year). Sputum and bronchial aspirate for microbiological investigation were obtained. Besides organisms commonly involved in bacterial infections of the lower respiratory tract (i.e. Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis), Enterobacteriaceae and Pseudomonas spp. were also found. A high percentage of these bacteria were beta-lactamase producers. These data along with the clinical presentation, severity of infection, and epidemiological knowledge, might represent a guide for the choice of empiric antimicrobial treatment.

Intensive care unit design and environmental factors in the acquisition of infection

The incidence of infection in the intensive care unit (ICU) is one of the highest in the hospital and yet facilities to prevent infection are often inadequate in this important clinical area. Many antibiotic-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA), Serratia marcescens and vancomycin-resistant enterococci (VRE), may survive and persist in the environment leading to recurrent outbreaks. A number of professional and scientific bodies in the UK, the USA and Europe have published guidelines on the design and layout of ICUs. All emphasize the importance of adequate isolation facilities (at least one cubicle for every six beds), sufficient space around each bed (20 m2), wash hand basins between every other bed, ventilation including positive and negative pressure ventilation for high risk patients and sufficient storage and utility space. Common sense and considerations of safety and comfort should guide decisions on floors, walls etc. Appropriate cleaning and disinfection programmes are essential to render the ICU relatively pathogen free and compliance with handwashing is imperative in minimizing infection in this high-risk area. Infection control teams should support ICU personnel in their efforts to upgrade facilities and help ensure that this is a priority when resources are limited.

Nosocomial infections in pediatric patients: a European, multicenter prospective study. European Study Group

OBJECTIVES

To determine the site and bacterial epidemiology of nosocomial infections (NIs) in children.

DESIGN

6-month prospective study with periodic chart review during hospitalization using a uniform prospective questionnaire in each unit, analyzed at a coordinating center.

SETTING

20 units in eight European countries: 5 pediatric intensive care units (PICUs), 7 neonatal units, 2 hematology-oncology units, 8 general pediatric units.

PARTICIPANTS

All children hospitalized during the study period with an NI according to Centers for Disease Control and Prevention criteria.

RESULTS

The overall incidence of NI was 2.5%, ranging from 1% in general pediatric units to 23.6% in PICUs. Bacteria were responsible for 68% (gram-negative bacilli, 37%; gram-positive cocci, 31%), Candida for 9%, and viruses for 22% of cases. The proportion of lower respiratory tract infections was 13% in general pediatric units and 53% in PICUs. Bloodstream infections were most frequent in neonatal units (71% of NIs) and were associated with a central venous catheter in 66% of cases. Coagulase-negative Staphylococcus (CNS) was the main pathogen. Eleven percent of NI were urinary tract infections. Gastrointestinal infections were most commonly viral and accounted for 76% of NIs in general pediatric units. The prevalence of antimicrobial resistance depended on the type of unit. The highest rates were observed in PICUs: 26.3% of Staphylococcus aureus and 89% of CNS were methicillin-resistant, and 37.5% of Klebsiella pneumoniae had an extended-spectrum beta-lactamase. Mortality due to NI was 10% in PICUs and 17% in neonatal units.

CONCLUSIONS

We found large differences in NI frequency and microbial epidemiology in this European study. Viruses were the main pathogens in general pediatrics units. Catheter-related sepsis and CNS were frequent in newborns. A high frequency of multiresistant bacteria was observed in some units. Clinical monitoring of NIs and bacterial resistance profiles are required in all pediatric units.

Bacteremia due to multiresistant gram-negative bacilli in neutropenic cancer patients: a case-controlled study

The aim of this study was to assess whether multiresistant gram-negative bacteremias (MRGNB) were associated with specific risk factors for higher mortality than sensitive gram-negative bacteremias. Two groups of subjects: (51 cases and 102 controls) were matched for sex, age, underlying disease, and neutropenia. There were no significant differences in the incidence of cytotoxic chemotherapy administered, vascular catheter insertion, catheter as source of bacteremia, and etiology of bacteremia. The proportion of Klebsiella-Enterobacter, Pseudomonas aeruginosa, Acinetobacter spp., and Stenotrophomonas maltophilia was similar in the two groups. Prior surgery (21.6% vs 7.9%, P < 0.05) was significantly associated with sensitive gram-negative bacteremia. Previous prophylaxis with ofloxacin (45.1% vs 24.5%; P < 0.05) and prior therapy with broad-spectrum antibiotics (41.2% vs 27.5%; P < 0.05), such as first and second generation cephalosporins (19.6% vs 7.8%; P < 0.05), third generation cephalosporins (41.2% vs 13.7%; P < 0.01), aminoglycosides (39.2% vs 9.8%; P < 0.01), ofloxacin (11.8% vs 2.0%; P < 0.005), and imipenem (19.6% vs 2.0%; P < 0.001) were significantly more frequently observed among cases than controls. Cases (patients with bacteremia due to multiresistant gram-negative bacteremias) were also significantly more frequently infected with bacteria resistant to ceftazidime (68.6% vs 17.6%; P < 0.001), amikacin (52.9% vs 7.8%; P < 0.001), imipenem (50.1% vs 23.5%; P < 0.05), ciprofloxacin (32.1% vs 5.9%; P < 0.001), and piperacillin (41.2% vs 7.8%; P < 0.01). With regard to outcome, attributable mortality was similar (15.7% vs 13.8%; not significant) in the two groups; however, cure rates were lower among cases (patients infected with MRGNB) because crude mortality was higher in cases (35.3% vs 13.8%; P < 0.01) than in controls.

Strategies for selecting antibiotics in intensive care units

Crowding of severely ill patients in intensive care units has led worldwide to important increases in nosocomial (ICU-related) infections. Moreover, the nature of these hospital-acquired infections is shifting towards Gram-positive microorganisms, yeast and Gram-negative rods, possessing important resistance genes (e.g. extended spectrum beta-lactamases and inducible Enterobacteriaceae). Ceftazidime and aztreonam are loosing their activity against the Gram-negative microorganisms. The fourth generation cephalosporins have an intrinsic high activity against the inducible Enterobacteriaceae. On our Hematology and Intensive Care units, the introduction of cefepime for nosocomial infections led to a remarkable drop in the number of Enterobacter isolates combined with important decreases in Enterobacter resistance towards several antibiotics.

Bacteremia due to multiresistant gram-negative bacilli in neutropenic cancer patients: a case controlled study

The aim of this study was to see if multiresistant Gram-negative bacteremias (MRGNB) are associated with specific risk factors and/or higher mortality in comparison to sensitive GNB (SGNB). Both groups, 51 patients and 102 controls, were matched for sex, age, underlying disease and neutropenia. In addition there were no significant differences in the incidence of cytotoxic chemotherapy administered, vascular catheter insertion and catheter as source of bacteremia and etiology of bacteremia. The proportion of Klebsiella-Enterobacter, Pseudomonas aeruginosa, Acinetobacter spp. and Stenotrophomonas maltophilia was similar in both groups. Prior surgery (21.6% vs 7.6%, p<0.02) was significantly associated with SGNB. Previous prophylaxis with quinolones (45.1% vs 24.5%, p<0.045), and prior therapy with broad spectrum antibiotics (41.2% vs 27.5%, p<0.05) were significantly more frequently observed among patients than controls. Patients with bacteremia due to MRGNB were also significantly more frequently infected with resistant bacteria. Attributable mortality was similar (15.7% vs 13.75%, NS) in both groups, however cure rates were lower among MRGNB patients. Crude mortality was higher among patients (35.3% vs 13.75%, p<0.01) in comparison to controls. In conclusion, prior antimicrobial prophylaxis and therapy with several classes of antimicrobials represents a significant risk for development of resistance. Mortality due to multiresistant Gram-negative bacteremias was higher in comparison to bacteremias due to susceptible organisms.