MDRO in ICU(Focus on Carbapenemase Producing Bacterias)

Introduction

The intensive care unit (ICU) often is called the epicenter of infections, due to its extremely

vulnerable population (reduced host defences deregulating the immune responses) and

increased risk of becoming infected through multiple procedures and use of invasive devices

distorting the anatomical integrity-protective barriers of patients (intubation, mechanical

ventilation, vascular access, etc.). In addition, several drugs may be administered, which also

predispose for infections, such as pneumonia, e.g., by reducing the cough and swallow

reflexes (sedatives, muscle relaxants) or by distorting the normal nonpathogenic bacterial

flora (e.g., stress ulcer prophylaxis)(Brusselaers et al, 2011)

Along with the problem of nosocomial infection goes the burden of “multidrug”

antimicrobial resistance (MDR). The ongoing emergence of resistance in the community and

hospital is considered a major threat for public health. Due to the specific risk profile of its

residents, the ICU also is deemed the epicenter of resistance development. The ICU has even

been described as a factory for creating, disseminating, and amplifying antimicrobial

resistance. (Brusselaers et al, 2011)

This burden of resistance, however, is probably more due to the higher rate of inappropriate

empiric antimicrobial treatment associated with infections caused by MDR pathogens than

with the virulence of particular MDR strains. (Brusselaers et al, 2011)

Important resistant pathogens

During the past decades, a shift in the MDR dilemma has been noted from gram-positive to

gram-negative bacteria, especially due to the scarceness of new antimicrobial agents active

against resistant gram-negative microorganisms. (Boucher et al, 2009)

Among gram-positive organisms, the most important resistant microorganisms in the ICU

are currently methicillin-(oxacillin-)-resistant Staphylococcus aureus, and vancomycin-

resistant enterococci (Boucher et al, 2009, Jones RN, 2001). In gram-negative bacteria, the

resistance is mainly due to the rapid increase of extended-spectrum Beta-lactamases (ESBLs)

in Klebsiella pneumonia, Escherichia coli, and Proteus mirabilis; high level third generation

cephalosporin -lactamase resistance among Enterobacter spp. and Citrobacter spp., and

MDR in Pseudomonas aeruginosa, Acinetobacter spp., and Stenotrophomonas maltophilia

(Jones RN, 2001)

Tabel 1. Multidrug-Resistant Bacterial Organisms Causing Major Clinical Problems. (Arias

and Murray, 2009)

The emergence of novel b-lactamases with direct carbapenem-hydrolyzing activity has

contributed to an increased prevalence of carbapenem resistant Enterobacteriaceae (CRE).

CRE are particularly problematic given the frequency with which Enterobacteriaceae cause

infections, the high mortality associated with infections caused by CRE, and the potential for

widespread transmission of carbapenem resistance via mobile genetic elements. (Gupta et al

2011).

Emergence and spread of carbapenem resistant Gram negative rods is a great concern,

especially in a resource limited country such as Pakistan, where treatment alternatives are

either unavailable or expensive/toxic with poor outcome. (Irfan et al, 2008)

Recent reports from our center have documented emergence and spread of carbapenem

resistance among multi-resistant non enterobacteriaceae including Acinetobacter species and

Pseudomonas aeruginosa. (Sarwari A, 2004; Noor A, 2005) Related trends in resistance

among P. aeruginosa have been observed in the national NNIS database (figure 3); among P.

Aeruginosa isolates recovered from ICU patients in 2003, the overall rate of resistance to

carbapenems was 20%, and that of resistance to third-generation cephalosporins and

quinolones was ∼30% (NNIS, unpublished data). ( McDonald, 2006)

Multidrug-Resistant Bacterial Organisms which cause major clinical problems are

summarized in table 1.

Mechanisms of resistance to Antibacterial agents

Bacteria may manifest resistance to antibacterial drugs through a variety of mechanisms.

Some species of bacteria are innately resistant to _1 class of antimicrobial agents. In such

cases, all strains of that bacterial species are likewise resistant to all the members of those

antibacterial classes. Of greater concern are cases of acquired resistance, where initially

susceptible populations of bacteria become resistant to an antibacterial agent and proliferate

and spread under the selective pressure of use of that agent. Several mechanisms of

antimicrobial resistance are readily spread to a variety of bacterial genera. First, the organism

may acquire genes encoding enzymes, such as _-lactamases, that destroy the antibacterial

agent before it can have an effect. Second, bacteria may acquire efflux pumps that extrude the

antibacterial agent from the cell before it can reach its target site and exert its effect. Third,

bacteria may acquire several genes for a metabolic pathway which ultimately produces

altered bacterial cell walls that no longer contain the binding site of the antimicrobial agent,

or bacteria may acquire mutations that limit access of antimicrobial agents to the intracellular

target site via downregulation of porin genes. Thus, normally susceptible populations of

bacteria may become resistant to antimicrobial agents through mutation and selection, or by

acquiring from other bacteria the genetic information that encodes resistance. The last event

may occur through 1 of several genetic mechanisms, including transformation, conjugation,

or transduction. Through genetic exchange mechanisms, many bacteria have become resistant

to multiple classes of antibacterial agents, and these bacteria with multidrug resistance

(defined as resistance to _3 antibacterial drug classes) have become a cause for serious

concern, particularly in hospitals and other healthcare institutions where they tend to occur

most commonly. As noted above, susceptible bacteria can acquire resistance to an

antimicrobial agent via new mutations.18 Such spontaneous mutations may cause resistance

by (1) altering the target protein to which the antibacterial agent binds by modifying or

eliminating the binding site (e.g., change in penicillin-binding protein 2b in pneumococci,

which results in penicillin resistance), (2) upregulating the production of enzymes that

inactivate the antimicrobial agent (e.g., erythromycin ribosomal methylase in staphylococci),

(3) downregulating or altering an outer membrane protein channel that the drug requires for

cell entry (e.g., OmpF in E coli), or (4) upregulating pumps that expel the drug from the cell

(efflux of fluoroquinolones in S aureus).18 In all of these cases, strains of bacteria carrying

resistance-conferring mutations are selected by antimicrobial use, which kills the susceptible

strains but allows the newly resistant strains to survive and grow. Acquired resistance that

develops due to chromosomal mutation and selection is termed vertical evolution.

Bacteria also develop resistance through the acquisition of new genetic material from other

resistant organisms. This is termed horizontal evolution, and may occur between strains of

the same species or between different bacterial species or genera. Mechanisms of genetic

exchange include conjugation, transduction, and transformation.18 For each of these

processes, transposons may facilitate the transfer and incorporation of the acquired resistance

genes into the host’s genome or into plasmids. During conjugation, a gram-negative

bacterium transfers plasmid-containing resistance genes to an adjacent bacterium, often via

an elongated proteinaceous structure termed a pilus, which joins the 2 organisms.

Conjugation among gram-positive bacteria is usually initiated by production of sex

pheromones by the mating pair, which facilitate the clumping of donor and recipient

organisms, allowing the exchange of DNA. During transduction, resistance genes are

transferred from 1 bacterium to another via bacteriophage (bacterial viruses). This is now

thought to be a relatively rare event. Finally, transformation, i.e., the process whereby

bacteria acquire and incorporate DNA segments from other bacteria that have released their

DNA complement into the environment after cell lysis, can move resistance genes into

previously susceptible strains.

Mutation and selection, together with the mechanisms of genetic exchange, enable many

bacterial species to adapt quickly to the introduction of antibacterial agents into their

environment. Although a single mutation in a key bacterial gene may only slightly reduce the

susceptibility of the host bacteria to that antibacterial agent, it may be just enough to allow its

initial survival until it acquires additional mutations or additional genetic information

resulting in fullfledged resistance to the antibacterial agent.18 However, in rare cases, a

single mutation may be sufficient to confer high-level, clinically significant resistance upon

an organism (e.g., high-level rifampin resistance in S aureus or high-level fluoroquinolone

resistance in Campylobacter jejuni). The following case studies, which involve 3 different

bacterial species, serve to illustrate several of the ways in which bacteria develop resistance

to antibacterial drugs and how different resistance mechanisms may interact to increase the

level or spectrum of resistance of an organism. (Tennover, 2006)

Carbapenem

Carbapenems are b-lactams that contain a fused b-lactam ring and a five-membered ring

system that differs from the penicillins in being unsaturated and containing a carbon atom

instead of the sulfur atom. This class of antibiotics has a broader spectrum of activity than do

most other b-lactam antibiotics. (Petri, 2006)

Imipenem is marketed in combination with cilastatin, a drug that inhibits the degradation of

imipenem by a renal tubular dipeptidase. The activity of imipenem is excellent in vitro for a

wide variety of aerobic and anaerobic microorganisms. Streptococci (including penicillin-

resistant S. pneumoniae), enterococci (excluding E. faecium and non-b-lactamase-producing

penicillin-resistant strains), staphylococci (including penicillinase-producing strains), and

Listeria all are susceptible. Although some strains of methicillin-resistant staphylococci are

susceptible, many strains are not. Activity is excellent against the Enterobacteriaceae,

including organisms that are cephalosporin-resistant by virtue of expression of chromosomal

or plasmid extended-spectrum b-lactamases. Most strains of Pseudomonas and Acinetobacter

are inhibited. X. maltophilia is resistant. Anaerobes, including B. fragilis, are highly

susceptible. (Petri, 2006)

Meropenem is similar to imipenem but has slightly greater activity against gram-negative

aerobes and slightly less activity against gram-positives. It is not significantly degraded by

renal dehydropeptidase and does not require an inhibitor. Ertapenem is less active than

meropenem or imipenem against P aeruginosa and acinetobacter species. It is not degraded

by renal dehydropeptidase.

A carbapenem is indicated for infections caused by susceptible organisms, eg, P aeruginosa,

which are resistant to other available drugs and for treatment of mixed aerobic and anaerobic

infections. Carbapenems are active against many highly penicillin-resistant strains of

pneumococci. A carbapenem is the -lactam antibiotic of choice for treatment of enterobacter

infections because it is resistant to destruction by the -lactamase produced by these

organisms; it is also the treatment of choice for infections caused by ESBL-producing gram-

negatives. Ertapenem is insufficiently active against P aeruginosa and should not be used to

treat infections caused by that organism. Imipenem or meropenem with or without an

aminoglycoside may be effective treatment for febrile neutropenic patients. (Katzung BG,

2006)

Doripenem has been shown to have potent inhibitory effects against P. aeruginosa isolates

(including difficult-to-treat strains), with a minimum inhibitory concentration that is 2– 4

times lower than that of imipenem and meropenem. Doripenem has demonstrated significant

efficacy among patients with a high Acute Physiology and Chronic Health Evaluation II

score and advantageous clinical outcomes in patients with difficult-to-treat gram-negative

infection. (Mandel, 2009)

Carbapenemase: classification and epidemiology

Carbapenemases are enzymes that can efficiently hydrolyse most β-lactams, including

carbapenems [4, 5]. (Paterson, 2005 and Queenan, 2007) Although known as

“carbapenemases,” many of these enzymes recognize almost all hydrolyzable -lactams, and

most are resilient against inhibition by all commercially viable β-lactamase inhibitors.

(Queenan, 2007)

The most commonly used classification for carbapenemases is that defined by Ambler,

although the one by Bush-Jacoby is also used. The Ambler classification separates β-

lactamases into four classes A-D, based on their molecular structure [5, 23]. Molecular

classes A, C, and D include the -lactamases with serine at their active site, whereas molecular

class B -lactamases are all metalloenzymes with an active-site zinc. Carbapenemases, -

lactamases with catalytic efficiencies for carbapenem hydrolysis, resulting in elevated

carbapenem MICs, include enzymes from classes A, B, and D. (Queenan, 2007)

Class A Carbapenemases

Class A carbapenemases are serine β-lactamases and contain serine at their active site (Bush,

2010). A variety of class A carbapenemases have been described; some are chromosome

encoded (NmcA, Sme, IMI-1, SFC-1), and others are plasmid encoded (Klebsiella

pneumoniae carbapenemases [KPC], IMI-2, GES, derivatives), but all effectively hydrolyze

carbapenems and are partially inhibited by clavulanic acid (2). (Nordmann,2010)

KPC is the most frequently encountered Class A carbapenemase and, along with its variants

KPC-2 to KPC-13, which differ solely by amino-acid mutations, it has spread throughout the

USA and globally [2]. The blaKPC gene is plasmid-mediated and is transported in a Tn3-

based transposon, Tn4401, which makes it readily transferable between bacterial isolates

[25].

Following the first report of a K. pneumoniae isolate harbouring blaKPC from USA [1],

blaKPC, spread efficiently with patient mobility and disseminated across borders

internationally [26-28]. KPCs are predominantly found in Enterobacteriaceae, most

commonly in K. pneumoniae isolates, but have recently also been reported in non-

fermentative bacteria such as Pseudomonas spp. [29, 30] and Acinetobacter spp. [31]

(ECDC, 2011). Figue 1 shows worldwide distribution of Klebsiella pneumoniae

carbapenemase (KPC) producers.

Figure 1. A) Worldwide geographic distribution of Klebsiella pneumoniae carbapenemase

(KPC) producers. Gray shading indicates regions shown separately: B) distribution in the

United States; C) distribution in Europe; D) distribution in China. (Nordmann,2010)

The level of resistance to carbapenems of KPC producers may vary markedly; ertapenem is

the carbapenem that has the lowest activity (5–7), (Table 2). KPC producers are usually

multidrug resistant (especially to all β-lactams), and therapeutic options for treating

KPCrelated infections remain limited (6) (Figure 2, panel A). Death rates attributed to

infections with KPC producers are high (>50%) (9–11). (Nordmann,2010)

Table 2. MIC range of carbapenems for Enterobacteriaceae that produce several types of

carbapenemases*

Class B Carbapenemases

Class B carbapenemases, also known as metallo-β-lactamases (MBLs), are zinc-dependent at

their active site. Originally, MBLs were described in non-fermentative bacteria such as

Pseudomonas spp. and Acinetobacter spp. [41, 42], and more recently have also been

described in Enterobacteriaceae [43, 44]. (ECDC, 2011).

Class B metallo-β-lactamases (MBLs) are mostly of the Verona integron–encoded metallo-β-

lactamase (VIM) and IMP types The first acquired MBL, IMP-1, was reported in Serratia

marcescens in Japan in 1991 (13). Since then, MBLs have been described worldwide (2,12)

(Figure 3). Endemicity of VIM- and IMP-type enzymes has been reported in Greece, Taiwan,

and Japan (2,12), although outbreaks and single reports of VIM and IMP producers have been

reported in many other countries (Figure 3). These enzymes hydrolyze all β-lactams except

aztreonam (12).Their activity is inhibited by EDTA but not by clavulanic acid (12). Most

MBL producers are hospital acquired and multidrug-resistant K.pneumoniae (2,12).

Resistance levels to carbapenems of MBL producers may vary (Table 1). Death rates

associated with MBL producers range from 18% to 67% (14). (Nordmann,2010).

In 2009, a novel MBL, the New Delhi MBL (NDM), was described [23, 24]. NDM was first

recognized in a K. Pneumoniae isolate from a Swedish patient who had received medical care

in India [24] and was soon recognized as an emerging mechanism of resistance in multiple

species of Enterobacteriaceae in the United Kingdom [23]. Many of the early cases in the

United Kingdom were associated with receipt of medical care in India or Pakistan [23, 25].

(Gupta et al 2011)

Kumarasamy et al (2010) found that among a convenience sample of Enterobacteriaceae

obtained from patients in India, between 31% and 55% of CRE isolates were NDM-producers

[25]. Many of the NDM-producing isolates from India were from patients with community-

onset infections.

Figure 2. Worldwide geographic distribution of Verona integron–encoded metallo-β-

lactamase (VIM) and IMP enterobacterial producers. (Nordmann,2010).