Is Selective Digestive Decontamination Useful in Controlling Aerobic Gram-Negative Bacilli Producing Extended Spectrum Beta-Lactamases?

Abstract

Aims: To identify outbreak episodes of either carriage or infection due to extended spectrum beta-lactamases producing aerobic Gram-negative bacilli (AGNB-ESBL); to establish whether AGNB-ESBL, sensitive to tobramycin, become resistant over time; and to evaluate the impact of selective decontamination of the digestive tract (SDD) on abnormal carriage of AGNB-ESBL. Design and Setting: All children admitted to the pediatric intensive care unit (PICU) over a 12-month period had biweekly surveillance cultures of throat and rectum and diagnostic cultures when clinically indicated. All AGNB were tested for ESBL, and the positive isolates were sent for molecular typing. The PICU uses SDD (parenteral cefotaxime and enteral polymyxin E/tobramycin) to control abnormal carriage. Patients who had at least one AGNB-ESBL were included in the study. Results: During the study period, 1,101 children were admitted to the PICU. There were 39 patients (3.5%) with a total of 236 cultures positive for AGNB-ESBL. Twenty-eight patients (2.5%) were carriers, and 11 (1%) had proven infections. Organisms isolated from the first culture were 14 patients with Klebsiella pneumoniae, 8 with Enterobacter cloacae, 7 with Citrobacter freundii, 5 with Klebsiella oxytoca, and 5 with Escherichia coli. In the first sample, 59% of isolates showed tobramycin resistance. Molecular typing confirmed that there were five different strains of K. pneumoniae and that similar strains were not isolated in the same period. Conclusions: SDD is an effective measure to control AGNB-ESBL and to avoid outbreak episodes of either carriage or infection. When tobramycin resistance is found, replacing it with another aminoglycoside based on antibiogram may be more effective in achieving AGNB clearance.

Introduction

The emergence of resistant micro-organisms is a rapidly evolving problem. This is particularly true in the setting of critically ill patients requiring treatment in intensive care units (ICU).¹³ After the introduction of extended spectrum beta-lactam antibiotics, for example, cefotaxime and ceftazidime in 1983, resistant aerobic Gram-negative bacilli (AGNB) emerged. Resistance occurred through a single amino acid change, the Gly238Ser substitution in the common penicillinases, TEM-1 or SHV-1, found in Escherichia coli and Klebsiella pneumoniae. A new beta-lactamase emerged, able to hydrolyze cefotaxime and ceftazidime.⁴ There are also other types (non-TEM and non-SHV) of class A extended spectrum beta-lactamases (ESBL) such as ceftazidimases—PER, VEB, TLA-1, GES/IBC—and cefotaximases—SFO-1, BES-1, CTX-M—whose frequency seems to be increasing.^4,6,19

Since their emergence, ESBLs have been the most important form of resistance to second- and third-generation cephalosporins. They occur mostly in Enterobacteriaceae (e.g., Es. coli, Klebsiella species and Enterobacter species) and rarely in nonfermenters (e.g., Pseudomonas aeruginosa).¹⁴ These resistant organisms are important, given the traditional therapeutic role of cephalosporins in intensive care. Many of the ESBL producers are also resistant to other commonly used antibiotics, including quinolones, and aminoglycosides, thus making the treatment of such patients a challenge.

Selective decontamination of the digestive tract (SDD) using parenteral and enteral antimicrobials, hygiene and topical antimicrobials has been shown to decrease carriage with resistant AGNB in intensive care units with a low prevalence of resistant pathogens and to improve survival.^10,12,17 However, the emergence of multi-resistant AGNB has been reported during SDD.^1,3

ESBL-producing AGNB (AGNB-ESBL) are now recognized as a widespread problem. Infection control programmes and strategies for antibiotic use are urgently needed to control this expanding form of resistance.²⁰

The epidemiology of resistant AGNB in pediatric ICUs (PICUs) is complex and varies from unit to unit.^28,33

Practically all parenteral antimicrobials have been identified as risk factors for acquisition, carriage, and subsequent overgrowth of AGNB-ESBL, particularly those agents that extensively damage the patient's gut flora.¹⁶

Aims

Introduction of AGNB-ESBL into the ICU often leads to dissemination, endemicity, and outbreaks of carriage/infection caused by AGNB-ESBL. We aim at establishing whether AGNB-ESBL brought into an ICU, which has introduced SDD into clinical practice, may lead to outbreaks.

AGNB-ESBL outbreaks are common in ICU. Most critically ill patients receive parenteral antimicrobials that suppress normal gut flora and promote abnormal carriage of AGNB-ESBL. SDD is a prophylactic maneuver using the enteral antimicrobials to eradicate existing abnormal carriage or prevent its future development. SDD including the combination of polymyxin E/tobramycin has been shown to rapidly eradicate an outbreak of an ESBL-producing Klebsiella.²⁶ Most AGNB-ESBL are sensitive to polymyxins and aminoglycosides, in particular to the high fecal concentrations achieved using enteral administration.

Endpoints

First end point

To identify outbreak episodes of either carriage or infection with AGNB-ESBL in a PICU using SDD over a 12-month period.

Second end point

To assess whether the AGNB-ESBL, initially sensitive to tobramycin, subsequently become resistant.

Third end point

To establish the efficacy of SDD in eradicating abnormal carriage and preventing infection with AGNB-ESBL.

Patients and Methods

Setting

Children admitted to the PICU at the Royal Liverpool Children's NHS Trust, Alder Hey, who had at least one sample positive for an AGNB-ESBL between October 1, 2005 and September 31, 2006 were included in the study. The hospital is a university-affiliated, multi-specialty, regional referral centre. The PICU is a 20-bedded facility, which includes eight isolation cubicles, with an annual admission rate of approximately 1,000 children. Cardiac and medical patients each account for 40% of the population, and 20% are surgical.

The Liverpool Children's Research Ethics Committee reviewed the study and waived the need for formal approval.

SDD policy

All patients with abnormal carrier states due to AGNB received SDD using parenteral cefotaxime and enteral polymyxin E (colistin)/tobramycin.²² Cefotaxime is included in SDD to cover primary endogenous infections due to common respiratory pathogens including Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae. Some patients who were very ill at admission or who had a condition that put them at great risk of infection, for example, patients with open chests after cardiac surgery, started the SDD regimen before the results of the surveillance cultures were available.

Detection of ESBL

As part of the SDD policy, surveillance cultures of the throat and rectum were obtained from all patients admitted to the PICU on admission and then twice weekly. Other cultures were performed when clinically indicated. All isolates of Enterobacteriaceae were tested for ESBL following the National Standard Method published by the Health Protection Agency (HPA), Collindale, London, United Kingdom.¹⁴ The isolates of the first positive culture were sent to HPA Collindale for molecular typing to differentiate the strains using pulsed-field gel electrophoresis.²⁷

Patients' characteristics

The records of all the patients included in the study were analyzed, and a database with the following characteristics was built: age, type of carriage/infection (see definitions), micro-organism(s) isolated, antibiotics used, type of nutrition (enteral/parenteral), SDD, past medical history (healthy, congenital heart disease, chronic lung disease, neuromuscular disorder, and others), and outcome.

Definitions

Normal flora

Normal floraexists when the surveillance samples yield indigenous aerobic and anerobic flora. Some people carry “normal” potential pathogens: Str. pneumoniae, H. influenzae, Moraxella catarrhalis, Sta. aureus, Candida albicans, and Es. coli.³¹

Abnormal flora

Abnormal flora exists when AGNB including Klebsiella, Enterobacter, Citrobacter, Proteus, Morganella, Serratia, Acinetobacter, Pseudomonas species, and/or methicillin-resistant Sta. aureus are persistently present in surveillance samples.²⁹

Polyclonality

The occurrence of different genotypes of a bacterial species.³⁰

Extended spectrum beta-lactamases

The term ESBL is used to describe acquired, class A β-lactamases that hydrolyse and confer resistance to oxyimino- “2nd- and 3rd-generation” cephalosporins, for example, cefuroxime, cefotaxime, ceftazidime, and ceftriaxone.⁴

Carriage or carrier state

The same strain of a micro-organism is isolated from two or more surveillance samples in a particular patient. Primary carriage is defined as carriage of a micro-organism detected in surveillance samples obtained on admission and within 3 days after arrival on the ICU, that is, the patient imported the strain in the admission flora. Secondary [super] carriage is defined as carriage of a micro-organism not present in the admission flora but acquired later on in the unit, in general after 5 days following admission to the unit.²¹

Colonization

Colonization is defined as the presence of a micro-organism in an internal organ that is normally sterile (e.g., lower airways, bladder), without an inflammatory response of the host.²¹

Infection

Infection is a microbiologically proven, clinical diagnosis of local and/or general inflammation. Infections were classified according to the concept of the carrier state.²¹

Primary endogenous is due to “normal” and/or “abnormal” micro-organisms present in the admission flora; usually occurring within one week; and the incidence is approximately 55% of all ICU infections.

Secondary endogenous is due to “abnormal” bacteria not present in the admission flora but acquired later on in the ICU. The “abnormal” bacteria are first acquired in the oropharynx, followed by carriage and overgrowth. Colonization and then infection may occur; generally after 1 week of ICU treatment; the frequency is about one-third of all ICU infections.

Exogenous infection is caused by “abnormal” bacteria introduced directly into the patient, omitting the stage of carriage, to a site where colonization and then infection occurs; can happen at any time during ICU treatment; accounts for approximately 15% of all ICU infections.

Surveillance cultures are defined as throat and rectal swabs to detect the “abnormal” carrier state; regularly obtained on Monday and Thursday.²⁹

SDD using parenteral and enteral antimicrobials, hygiene and topical antimicrobials is an antibiotic prophylaxis aiming at the control of primary and secondary endogenous infections and exogenous infections, due to 15 “normal” and “abnormal” potential pathogens and at the reduction in mortality due to infection.²²

Outbreak

An event in which two or more patients in a defined location are infected by identical micro-organisms, usually within an arbitrary time period of 2 weeks. Using the surveillance culture approach two different types of infection, secondary endogenous and/or exogenous, can cause outbreaks.²¹

Resistance to tobramycin

AGNB with a minimum inhibitory concentration for tobramycin greater than or equal to 8 mg/L.²⁸

Resistance to polymyxin

AGNB with a minimum inhibitory concentration for polymyxin greater than or equal to 4 mg/L.²⁸

Results

During the study period, 1,101 children were admitted to the PICU. There were 39 patients (3.5%) with a total of 236 positive cultures for ESBL organisms (Table 1). Twenty-eight patients (2.5%) were carriers, and 11 (1%) had proven infections. The resistant organisms isolated in the first culture were K. pneumoniae in 14 patients (35.9%), Enterobacter cloacae in 8 (20.5%), Citrobacter freundii in 7 (17.9%), Klebsiella oxytoca in 5 (12.8%), and Es. coli in 5 (12.8%).

Table 1.

Database Excerpt

ID	AGNB-ESBL 1	Tobramycin resistant	AGNB-ESBL 2	Carriage	Infection	Cleared AGNB-ESBL	Outcome
1	Klebsiella oxytoca	Yes	C. freundii	Yes (C. freundii)	Blood (K. oxytoca)	No (K. oxytoca)	Died
2	Enterobacter cloacae	Yes	C. freundii	Yes	No	Yes	Died
3	Escherichia coli	No	K. pneumoniae	Yes	No	Yes	Ward
4	En. cloacae	Yes		Yes	Chest	No	Died
5	Es. coli	Yes		Yes	No	N/A	Ward
6	Klebsiella pneumoniae	Yes		Yes	Blood, Chest	No	Died
7	Citrobacter freundii	No		Yes	No	Yes	HDU
8	K. pneumoniae	Yes	K. ozaenae	Yes	Blood	No	Died
9	K. oxytoca	No		Yes	No	Yes	Ward
10	K. pneumoniae	No		Yes	No	Yes	Ward
11	En. cloacae	Yes		Yes	Abdominal wound, mediastinum	No	Died
12	K. pneumoniae	Yes		Yes	No	Yes	HDU
13	K. pneumoniae	Yes		Yes	Chest	Yes	HDU
14	K. pneumoniae	Yes	En. cloacae	Yes	Chest	N/A	Home
15	K. pneumoniae	Yes	K. pneumoniae spp2	Yes	No	N/A	Died
16	En. cloacae	No	C. freundii/K. pneumoniae	Yes	No	Yes	NICU
17	K. oxytoca	No		Yes	No	Yes	Ward
18	Es. coli	No	K. pneumoniae	Yes	No	Yes	HDU
19	En. cloacae	Yes		Yes	No	N/A	HDU
20	K. pneumoniae	Yes	En. cloacae	Yes	No	Yes	HDU
21	C. freundii	No	K. pneumoniae	Yes	No	Yes	HDU
22	En. cloacae	No		Yes	No	Yes	Ward
23	C. freundii	No	K. pneumoniae	Yes	No	Yes	HDU
24	K. oxytoca	No		Yes	No	N/A	Ward
25	K. pneumoniae	Yes		Yes	Chest	Yes	HDU
26	Es. coli	No	K. oxytoca	Yes	No	N/A	HDU
27	K. pneumoniae	Yes	Es. coli	Yes	Chest	Yes	HDU
28	C. freundii	No	En. cloacae	Yes	No	N/A	HDU
29	Es. coli	No		Yes	No	N/A	Ward
30	En. cloacae	No		Yes	No	Yes	Ward
31	K. pneumoniae	Yes		Yes	No	N/A	Ward
32	K. pneumoniae	Yes	En. cloacae	Yes	Chest	Yes	Home
33	C. freundii	Yes	K. pneumoniae	Yes	Blood	No	Died
34	C. freundii	Yes	K. pneumoniae	Yes	No	Yes	HDU
35	En. cloacae	Yes	En. cloacae spp2	Yes	No	Yes	HDU
36	K. pneumoniae	Yes	K. oxytoca	Yes	No	N/A	Home
37	K. oxytoca	Yes		Yes	No	N/A	HDU
38	C. freundii	No		Yes	No	N/A	Ward
39	K. pneumoniae	Yes		Yes	No	Yes	Ward

N/A, the patient was discharged before clearance could be tested; HDU, High Dependency Unit; NICU, Neonatal Intensive Care Unit; AGNB-ESBL, Gram-negative bacilli-extended spectrum beta-lactamase.

In the 11 patients infected with AGNB-ESBL, there were six lower airway infections, four blood-stream infections, and one wound infection. According to the concept of the carrier state, there were two primary endogenous, six secondary endogenous, and three exogenous infections. The patients who developed exogenous infections subsequently became carriers. This means that all 39 patients became carriers of AGNB-ESBL either before or after being infected. Twenty-three patients had positive rectal cultures, and 16 had positive throat and rectal cultures.

The past medical history of the patients included in the study revealed that the majority (82%) suffered from chronic disorders. Nineteen patients had congenital heart disease, six had other congenital malformations, five were on long-term ventilation, and two were immunosuppressed.

Twenty-three patients out of 39 patients (59%) had isolates that showed tobramycin resistance (tobramycin is a component of the SDD regimen). There were no documented failures in clearing the AGNB-ESBL in the 16 patients with tobramycin-sensitive AGNB-ESBL. Eleven cleared the AGNB-ESBL, and five were discharged before cultures were repeated. None became infected. Only two patients whose AGNB-ESBL were sensitive to tobramycin subsequently acquired a different resistant AGNB-ESBL strain. No patients in the tobramycin-sensitive group died compared with eight patients who died in the tobramycin-resistant group (0% vs. 35%, p = 0.01). On the other hand, 9 of the 23 (39%) patients carried tobramycin-resistant AGNB-ESBL where SDD failed to clear the resistant strain. Of these nine patients, seven died.

Twenty-two (56%) patients were already carriers of AGNB-ESBL at the time of admission to PICU. Twenty patients (51%) started preemptive SDD before the results of the surveillance cultures were known.

Except for one K. pneumoniae misclassified as K. Oxytoca, all the other bacteria were correctly classified by conventional methods. Molecular typing also allowed differentiation of the strains using pulsed-field gel electrophoresis. The Enterobacter and the Citrobacter all represented different strains. Among the Klebsiellae, there were five different strains and similar strains were not isolated in the same period (Table 2).

Table 2.

Classification of the Isolates Sent for Molecular Typing and Determination of the Different Strains

ESBL Gram negative bacteria	Molecular typing classification	Serotype	PFGE/Strain
K. pneumoniae	K. pneumoniae	Non typable	RLIV03KL-3
K. oxytoca	K. pneumoniae	K3/K68	Unique
K. pneumoniae	K. pneumoniae	Non typable	RLIV03KL-1
C. freundii	C. freundii		Unique
K. pneumoniae	K. pneumoniae	K28	RLIV03KL-2
En. cloacae	En. cloacae	01/03	Unique
K. pneumoniae	K. pneumoniae	Non typable	RLIV03KL-4
K. pneumoniae	K. pneumoniae	Non typable	RLIV03KL-1
K. pneumoniae	K. pneumoniae	Non typable	RLIV03KL-3
C. freundii	C. freundii	Non typable	Unique
En. cloacae	En. cloacae	01/03	Unique
K. pneumoniae	K. pneumoniae	Non typable	RLIV03KL-4
K. pneumoniae	K. pneumoniae	Non typable	RLIV03KL-2
K. pneumoniae	K. pneumoniae	Non typable	RLIV03KL-1

PFGE, pulsed field gel electrophoresis.

AGNB-ESBL were successfully cleared in 21 patients (54%). In some patients, clearance was only achieved after peristalsis returned after weeks in PICU.

Regarding outcome, eight patients died (21%) and most of the others were discharged to a High Dependency Unit or a regular ward. From the eight patients who died, six were infected with AGNB-ESBL and two were carriers.

There was no outbreak of either carriage or infection during the study period.

Discussion

The answer to the question whether SDD is useful in controlling AGNB-ESBL is yes, because of two reasons:

SDD prevented outbreak episodes of carriage or infection with AGNB-ESBL.

Tobramycin-sensitive AGNB-ESBL were effectively cleared without becoming resistant.

There was no outbreak of carriage or infection due to AGNB-ESBL in our PICU over a 12 month period using a SDD strategy. Our observation that there were no outbreaks during this observational cohort study is inline with a randomized controlled trial (RCT) assessing the impact of enteral antimicrobials on endemicity of an ESBL-producing K. pneumoniae. Carriage and infection rates due to the ESBL-producing K. pneumoniae were 19.6% and 9%, respectively, in the control group. Once enteral antimicrobials were added to the parenteral, there was a significant reduction in both carriage and infection (19.6% vs. 1%; 9% vs. 0%).⁸

The prevalence of both carriage and infection with AGNB-ESBL was low in our study. Molecular typing confirmed that although there were patients with similar strains of K. Pneumoniae, there was no outbreak because these patients were not related in time or bed location within the PICU. More than half of the AGNB-ESBL carriers (56%) were carrying the strain at the time of admission.

Most patients (82%) who developed secondary carriage and/or were infected with AGNB-ESBL were severely ill patients with chronic conditions such as congenital malformations, neuromuscular disorders on long-term ventilation, or immunosuppression. This is not surprising but reinforces the need for strategies to protect this particular group of patients from acquiring carriage and possible infection with multiresistant organisms. Some of these patients have multiple admissions to PICU and they are at risk of being discharged to other wards or the community carrying the AGNB-ESBL if they are not cleared while in the PICU.

There were no cases of tobramycin-sensitive AGNB becoming resistant, although two patients acquired different strains of AGNB-ESBL. This may be explained by the use of SDD that prevents polyclonality by eradicating overgrowth. The gut of the critically ill has been suggested as a site of microbial overgrowth with increased spontaneous mutations leading to emergence of new clones.³⁰ We believe that SDD prevents this overgrowth and the appearance of resistant clones. This statement is in line with the literature demonstrating that resistance has never been a major clinical problem during 25 years of clinical SDD research. Data from 56 RCTs and 12 meta-analyses do not provide any evidence for a link between SDD and the emergence of antimicrobial resistance.²⁴ Antimicrobial resistance, being a long-term issue, has been evaluated in seven SDD studies of minimally 2 years. Bacterial resistance associated with SDD has not been a clinical problem.^{9,11,15,18,22,25,32}

One finding that emerges from our study is that SDD failed to clear abnormal carriage of tobramycin-resistant AGNB-ESBL in a setting with a substantial level of tobramycin resistance (59% of AGNB-ESBL cultures). It is highly likely that there was polyclonality before admission. These children were seriously ill and their condition could be favoring gut overgrowth as just described. Some of these patients had been previously admitted to the PICU and had already been on an SDD regimen that could theoretically promote resistance to tobramycin. Nonetheless, there were at least two patients transferred from other hospitals without an SDD policy who were carriers of tobramycin-resistant strains at the time of admission, showing that this is probably a more generalized problem. Our focus should be on strategies to prevent carrier states with resistant AGNB and, if that is not possible, to clear the AGNB carrier state before the patient is infected. The high tobramycin-resistance rate represents an important problem, because SDD failed to clear these AGNB-ESBL effectively. Brun-Buisson describes endemicity of carriage and infection due to an ESBL-producing K. pneumoniae resistant to tobramycin. The outbreak strain was sensitive to 32 mg of neomycin/L.²⁴ A similar problem, where a Serratia strain was resistant to polymyxin E and tobramycin, was dealt with by the ICU team in Getafe (Madrid) by replacing both by paromomycin.⁵ Endemicity was controlled within a week (unpublished data).

Our study has some limitations. We did not evaluate any alternative strategies to control AGNB-ESBL. We did not study the resistance genes in different species to establish whether there are identical plasmids with the same ESBL genes. Therefore, we cannot be sure that there was no plasmid transfer between the bacteria despite them being of different species.

We think our study poses many important questions and gives some clues about the strategies to minimize the inevitable problem of antimicrobial resistance. We realize that most PICU do not have a SDD policy and our results cannot be extrapolated to different settings. SDD is a safe and effective method of decreasing PICU infection and mortality. Three recent, RCTs in adult ICU^10,12,17 support this view. Nonetheless, there is scope to improve the formulation of SDD to achieve better outcomes. One strategy could be to tailor the formulation according to resistant patterns.

In conclusion, we believe that the clinical issue is not the mechanism of ESBL resistance in AGNBs, whether it is plasmid- or chromosome-related,^2,23 but the inability of the patient to clear carriage of AGNB-ESBL if resistant to tobramycin.⁷

Conclusion

SDD is an effective measure to control AGNB-ESBL and to avoid outbreak episodes of either carriage or infection. When tobramycin resistance is found, replacing it with another aminoglycoside based on antibiogram may be more effective in achieving AGNB clearance.

Footnotes

Acknowledgments

We wish to thank Mrs M.E. Kaufmann, Head of Opportunist Pathogens Services, Laboratory of Health Care Associated Infections, Specialist and Reference Microbiology Division, HPA Collindale, London, United Kingdom, for the valuable work in molecular typing of the isolates. We are grateful to Dr. V. Damjanovic for carefully reviewing the manuscript. No financial support was used for the present study.

Disclosure Statement

The authors have nothing to disclose. All authors state that they have no conflicts of interest according to the journal policy, either actual or potential.

Name of the institution where the study was performed: Royal Liverpool Children's NHS Trust–Alder Hey Hospital.

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