Pseudomonas aeruginosa infections in the neonatal intensive care unit

Pseudomonas aeruginosa infections in the neonatal intensive care unit

Pseudomonas aeruginosa Infections in the Neonatal Intensive Care Unit Marc D. Foca Pseudomonas aeruginosa is a highly adaptable gram-negative bacillu...

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Pseudomonas aeruginosa Infections in the Neonatal Intensive Care Unit Marc D. Foca

Pseudomonas aeruginosa is a highly adaptable gram-negative bacillus with the ability to cause serious disease in vulnerable populations. This article reviews the relevant epidemiology of this pathogen in the hospital setting with particular attention to the neonatal unit. Issues related to reservoirs o f the organism with special consideration o f the hands of staff are also addressed. Virulence factors and pathogenic mechanisms are highlighted as well as the important role of anfimicrobial resistance patterns. Finally, there is a discussion of the clinical syndromes found in neonates and the appropriate antibiotic usage strategies for effective treatment of this pathogen of continuing importance. Copyright 2002, Elsevier Science (USA). All rights reserved. seudomonas aeruginosa is a ubiquitous gramnegative bacillus with the ability to survive in harsh environments not suited to o t h e r bacteria. 1 It is considered an opportunistic pathogen, because it rarely causes serious disease in the n o r m a l host; however, it is a cause of hospital-acquired infections and infections in those persons with serious underlying medical conditions including p r e t e r m infants. 2 T h e improved survival of smaller and y o u n g e r neonates has resulted in an increased rate of hospitalacquired infections. ~ Infants in the neonatal intensive are unit (NICU), especially very low birth weight (VLBW) infants, are at increased risk of infection f r o m a variety of developmental and environmental causes (reviewed in Saiman, this issue). These include a m o r e fragile skin barrier, an i m m a t u r e i m m u n e system essentially devoid of maternal antibody, p r o l o n g e d use of invasive devices, and colonization with antibiotic experie n c e d hospital flora. Although it remains a relatively u n c o m m o n p a t h o g e n , P aeruginosa has the ability to cause extensive morbidity a n d rapid mortality even when diagnosed swiftly in this patient population. 4 p

Epidemiology

From the Division of InJectious Diseases, Department of Pediatrics, Columbia University, New York, NY. Address reprint requests to Marc D. Foca, MD, Columbia University, Division of Infectious Diseases, Department of Pediatrics, Children's Hospital of New York, 622 l~ 168th St, VC-4East Rm. 449H, New York, NY 10032; e-mail: mdfl [email protected] Copyright 2002, Elsevier Science (USA). All rights reserved. 0146-0005/02/2605-0004535.00/0 doi:l O.1053/spo: 2002.36266

Early reports of Pseudomonas infections in neonatal units b e g a n to surface in 1960. 5 T h e emergence of gram-negative pathogens as a m a j o r cause of neonatal disease occurred with the advent of anfimicrobials capable of suppressing gram-positive organisms. 6 Traditional reservoirs for this p a t h o g e n include sinks, 7 water b a t h s / respiratory e q u i p m e n t , 8 neonatal incubators, 9 and h a n d lotions, a~ However, P aeruginosa can be isolated f r o m such sites as walls, floors, and phototherapy equipment, ll With the advent of large-scale, prospective surveillance for hospital-acquired infections, it has b e e n possible to track the incidence of various nosocomial infections by site and by organism to assess their relative i m p o r t a n c e for control purposes. T h e National Nosocomial Infections Surveillance System (NNISS) has b e e n in place for almost two decades. It collects prospective data f r o m 99 hospitals with neonatal units using standard definitions for a select g r o u p of hospital-acquired infections. While the full network looks at infections across a b r o a d range of patient populations and types of patient units, a s u m m a r y of the data for neonatal units was published in 1996.12 This r e p o r t offers the most complete baseline data for this patient population, and it establishes the b e n c h m a r k by which individual units can c o m p a r e themselves. T h e data also has the benefit of using birth weight categories to better assess risk for different types of infections in these distinct patient populations. Bloodstream infections were the most c o m m o n cause of hospital-acquired infection at all weight ranges comprising 32% to 49%

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Pseudomonas Infections in the NICU

of all infections reported. P n e u m o n i a and ear, eye, nose, and throat (EENT) infections were the second and third most c o m m o n hospitalacquired infections. P aeruginosa was a relatively infrequent pathogen and accounted for 6.6% of EENT infections and 11.7% of pneumonias. It was not r e p o r t e d at all as a bloodstream p a t h o g e n or a surgical site infection in this cohort. Similarly, P aeruginosa was a rare cause o f bloodstream infection in a prospective study at a single institution. 13 Over a 12-year period, only 2% of bloodstream infections were caused by P aeruginosa. These overall data are useful for comparison purposes, but they do not predict risk factors for infection. T h e Pediatric Prevention Network (PPN), under the auspices of the Centers for Disease Control (CDC) and the NationaI Association o f Children's Hospitals and Related Institutions (NACHRI), published a point prevalence study of hospital-acquired infections in neonatal units that begins to address the issue o f risk factors in this patient p o p u l a t i o n ) 4 As with the NNISS data, bloodstream infections, p n e u m o n i a , and EENT infections were the most c o m m o n infections. Overall risk factors in this population included lower birth weight, longer duration of stay, central venous catheter use, and use of total parenteral nutrition. P aeruginosa r e m a i n e d a relatively u n c o m m o n infection and accounted for 5% of bloodstream infections, 15% o f pneumonias, and 11% of EENT infections. However, these data fail to predict the severity of individual infections. A recent case control study places the importance of P aeruginosa as a nosocomial p a t h o g e n in better perspective. Leigh et a115 c o m p a r e d 22 neonates with P aeruginosa infection (predominately bloodstream) to 44 control infants m a t c h e d for gestational age, birth weight, sex, and duration o f stay. All infections occurred after 5 days of age and in infants that weighed <1500 g at birth. Clinically, the investigators were unable to distinguish P aeruginosa infection from o t h e r neonatal infectious episodes, and 50% of the patients died. Risk factors included feeding intolerance, p r o l o n g e d parenteral nutrition, longer duration of intravenous antibiotics, and necrotizing enterocolitis. T h e investigators n o t e d that all infants who were infected in the first week of life died and that mortality was inversely related to age at diagnosis. Over half

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the deaths occurred within 24 hours of presentation. These results highlight the i m p o r t a n c e of this p a t h o g e n in VLBW infants. F u r t h e r m o r e , infants treated with 2 anti-pseudomonal agents had better outcomes; while 21 o f 22 infants were empirically treated with an anti-pseudomonalaminoglycoside, 8 of 11 infants who received a second d r u g with activity against P aeruginosa survived whereas 2 of 11 infants who received only 1 a g e n t sur~ived. A m o r e recent report o f an outbreak of P aeruginosa in a NICU confirmed the high mortality rate from this p a t h o g e n (35%) and implicated artificial nails worn by healthcare workers as a potential reservoir for this infectious agent.l~

Hand Colonization Normal hand flora is c o m p o s e d of transient and resident organisms.IV Transient flora is easily rem o v e d through hand washing a n d / o r disinfection, and the resident flora remains in the openings o f the hair follicles and the sebaceous glands. T h e most c o m m o n h a n d organisms are micrococci, streptococci, and corynebacterium. Gram-negative bacilli have traditionally b e e n t h o u g h t to be less c o m m o n as p r o l o n g e d residents of the hand flora; however, this view may be changing. Reports of the hands of personnel acting as vectors for the transmission of gram-negative organisms surfaced as early as 1967, ~8 but little was known a b o u t the ability o f these organisms to multiply on hands or for hands to act as a vehicle for transmission. Knittle et a119 cultured the hands of personnel during an outbreak of enteric pathogens with kanamycin resistance in their NICU. These investigators found a high rate of colonization with enteric pathogens in general (86%) and with kanamycin resistant organisms in particular (48%). This g r o u p also found that while the distribution of most organisms tended to be random, particular staff carried specific organisms with high frequency even after they had been away f r o m the unit for a p r o l o n g e d period of time. Switching hand antisepsis products reduced the isolation of gramnegative pathogens over time. This study showed that gram-negative organisms can be m o r e than transient residents of the normal h a n d flora, and it suggested that periodic alteration of h a n d antiseptics could control the n u m b e r of such

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isolates and potentially reduce the nosocomial transmission rate. Adams a n d Marrie 2~ e x t e n d e d these observations by c o m p a r i n g the flora on the hands of healthcare workers with that of staff that did not have patient contact. T h e hands of patient care staff were m o r e likely to h a r b o r traditional nosocomial p a t h o g e n s such as Klebsiella, Pseudomonas, and Citrobacter spp. In addition, these investigators showed that healthcare workers could continuously h a r b o r gram-negative organisms on their hands and that staff with overt h a n d dermatitis had greater n u m b e r s of organisms and an increased frequency of colonization. Additional evidence confirming these observations has b e e n provided by o t h e r investigators, m, 22 Larson 23 d o c u m e n t e d that gram-negative pathogens persisted on the nails of p e r s o n n e l after a surgical scrub. Subsequent work 24 revealed that staff who wore artificial nails were m o r e likely to h a r b o r gram-negative pathogens b o t h before and after h a n d cleansing then were staff with natural nails. This latter study also showed that those with artificial nails had higher colony counts of bacteria than did those with natural nails. T h e majority of the evidence indicates that gram-negative bacilli including P aerug~nosa can be m a i n t a i n e d as n o n t r a n s i e n t flora of the hands of medical personnel and that artificial nails increase the quantity and the resiliency of these organisms. T h e hands of staff must be considered potential reservoirs during any o u t b r e a k investigation. This has b e e n shown for P aeruginosa on several occasions. W i d m e r et al z5 linked an o u t b r e a k of P aeruginosa in a surgical intensive care unit to the hands of a single staff m e m b e r . Similarly, Foca et a126 d o c u m e n t e d an increased risk of Pseudomonas acquisition in neonates after exposure to a staff m e m b e r colonized with the same clone as determ i n e d by pulsed field gel analysis. O f note, these investigators d o c u m e n t e d not only artificial nails, but also increasing age of staff as risk factors for h a n d colonization with this organism.

Epidemiologic Summary P aeruginosa has b e e n recognized as a neonatal p a t h o g e n for almost 50 years. Multiple reservoirs have b e e n d o c u m e n t e d , including patient care e q u i p m e n t and the environment. While a

m i n o r p a t h o g e n as a p e r c e n t a g e of all nosocomial infections, it has an unacceptably high mortality rate in affected infants. Risk factors for acquisition include: VLBW, p r o l o n g e d antibiotic usage, feeding intolerance, use of total parenteral nutrition, and necrotizing enterocolitis. H a n d s of p e r s o n n e l should be considered potential reservoirs of transmission during an outb r e a k investigation especially when m o r e traditional reservoirs fail to grow the pathogen. T h e use of artificial nails by N I C U staff should be forbidden.

Organism As stated previously, P aeruginosa is a motile, aerobic, gram-negative rod. It is capable of producing several different pigments including pyocyanin, which gives the bacterial colonies their characteristic b l u e / g r e e n tint when cultured. This p i g m e n t is only p r o d u c e d in a b o u t half of all P aeruginosa isolates, but it is characteristic of this organism as is the "grape-like" odor. 27 W h e n present, these features distinguish P aeruginosa f r o m non-aeruginosa isolates of Pseudomonas. It is capable of using 30 or m o r e different organic c o m p o u n d s as nutritional sources, and thus, it can maintain itself u n d e r diverse and harsh conditions. 27 T h e organisms' resiliency as a pathogen is a c o m b i n a t i o n these nutritional capabilities as well as its i n h e r e n t resistance to antimicrobials a n d a multitude of virulence factors. T h e pathogenesis of any microbe d e p e n d s on several factors including, a t t a c h m e n t to the appropriate host cell, colonization, local invasion a n d p r o d u c t i o n of virulence factors, and dissemination with eventual systemic manifestations. Protein structures on the surface of Pseudomonas, pill a n d fimbriae are necessary for a t t a c h m e n t to epithelial surfaces. 2s In addition, disruption of the epithelial tight junctions probably aids in the a t t a c h m e n t process. 29 O n c e the epithelium has b e e n colonized, some of the organisms will produce a biofilm c o m p o s e d of a m u c o i d exopolysaccharide called alginate. ~~This occurs by a c o m p l e x process involving the secretion of small molecules resulting in bacterial c o m m u n i c a t i o n t e r m e d " q u o r u m sensing. T M Both gram-positive a n d gram- negative organisms use this m e t h o d of communication, but gram-positive organisms use small peptides whereas Gram-negative organisms use chemicals ( h o m o s e r i n e lactones).

Pseudomonas Infections in the NICU

The molecules are expressed at low levels constitutively, and cell-to-cell signaling requires a threshold density o f organisms. The result is the coordination of expression of sets o f genes involved in biofilm formation and virulence. Strains that produce biofilms may be less susceptible to antimicrobial action, 32 and this may play a role in the pathogenesis of catheter infections as well as pulmonary disease caused by this organism. 33 Virulence is associated with the production of multiple proteases 34 such as elastase. These substances cleave a wide variety of host molecules including collagen, IgA, IgG, and complement, and they may be important in the disruption of epithelial tight junctions m e n t i o n e d previously. -~5 P aerug~nosa also produces a protein called cytotoxin, which may be capable o f inhibiting polymorphonuclear lymphocyte function. 36 It also produces heat labile and heat stable hemolysins that act in concert to degrade host lipids. 27 Many of these products not only participate in local invasion, but may also be involved in dissemination to other organs. Systemic toxicity, however, is mediated by several novel protein products. Exotoxin A is a potent inhibitor of protein synthesis that works in the same fashion as the diphtheria toxin (binding to elongation factor 2 of the protein synthesis complex). 37 This protein may also play a role in i m m u n e dysregulation.38, 39 Exoenzyme S disrupts actin filaments and is essentially injected into the host cell through a type III secretion system. 4~ These systems have been f o u n d in many bacteria and appear to be involved in the delivery of virulence related proteins into the host cell. 4~ Several proposed mechanisms exist and include: coupled translation and secretion of protein across the bacterial membranes via a channel, secretion of protein with an amino terminal signal sequence, and protein chaperones that facilitate entry into the m e m b r a n e spanning channel 42 (Fig 1). As with all gram-negative bacteria, P aeruginsoa possesses endotoxin, a major mediator of the sepsis syndrome caused by these organisms.

Antibiotic Resistance P a~ginosa is composed of an outer lipid bilayer, a periplasmic space, and an inner bacterial m e m b r a n e (Fig 2). This complex structural

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o' 'e Figure 1. Proposed mechanisms for type III secretion systems. (A) Coupled translation and secretion of virulence factors. (B) Signal sequence-induced secretion of proteins. (C) Chaperone-mediated secretion of proteins. (Reprinted from Aldridge P, Hughes KT: How and when are substrates selected for type III secretion. Trends Micro 9:209-214, 2001 with permission of Elsevier Science.42) arrangement allows the organism to maintain multiple mechanisms of resistance to natural defensins (components of the innate i m m u n e system) and antimicrobial agents. Probably the most important such mechanism involves the ability to p u m p antibiotics out of the periplasmic space (Fig 3). T h e mexAB o p e r o n efflux system acts as a major cause of resistance to multiple classes o f antibiotics including, macrolides, tet-

Marc D. Foca

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Figure 2. Membrane structures of various bacterium. The membrane structures of gram-positive cocci (left), gram-negative bacilli (center), and mycobactefia (right) are shown, indicating the progressive complexi W of these structures. (Reprinted with permission from Nikaido H: Prevention of drug access to bacterial targets: Permeability barriers and active efflux. Science 264:382-387, 1994. Copyright 2002 American Association for the Advancement of Science. 51)

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ses that have h i g h e r affinity for the penicillins. 43 This class o f beta lactamase can m u t a t e into an e x t e n d e d s p e c t r u m beta lactamase with enh a n c e d activity against the third g e n e r a t i o n cephalosporins. These plasmid associated elem e n t s are often co-expressed with o t h e r resistance m e c h a n i s m s i n c l u d i n g aminoglycoside m o d i f y i n g enzymes that r e n d e r the o r g a n i s m multidrug-resistant.

racyclines, t r i m e t h o p r i m , c h l o r a m p h e n i c o l , a n d f l o u r o q u i n o l o n e s . 43 This efflux system, its h o m o logues, a n d the p r o d u c t i o n o f beta lactamases, make P aeruginosa a f o r m i d a b l e p a t h o g e n to treat in the clinical setting. Pseudomonas expresses the p r o d u c t o f the ampC gene, a chrom o s o m a l beta lactamase. 4-~ It is n o r m a l l y expressed at low levels a n d has greatest affinity for the cephalosporins; however, m u t a n t s with constitutive p r o d u c t i o n can also inactivate penicillin derivatives such as piperacillin a n d ticarcillin that n o r m a l l y have a n t i - p s e u d o m o n a l activity. O r g a n i s m s that express this beta lactamase are often sensitive to the aminoglycosides a n d the f l o u r o q u i n o l o n e s unless the o r g a n i s m has b e e n u n d e r e x t r e m e selective pressure such as can o c c u r in the ICU. P aeruginosa can also expresss, a l t h o u g h less frequently t h a n o t h e r gram-negative p a t h o g e n s , plasmid associated beta lactama-

Clinical Syndromes

P aeruginosa causes a wide array o f c o m m u n i t y a c q u i r e d clinical infections including, keratitis, endocarditis, otitis externa, osteomyelitis, a n d folliculitis. 97 However, in the hospitalized neonate, a smaller n u m b e r o f well-recognized clinical infections are the n o r m . Pseudomonas is a m a j o r cause o f n o s o c o m i a l p n e u m o n i a , urinary

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protein efflux system to ba,n__a h omp.m remove antimicobials once they have managed to pass through outer membrahe, the periplasmic space. and the inner membrane of the organism. (Reprinted with permission from Nikaido H: Prevention of drug access to bacterial targets: Permeability barriers and active effltLx. Science 264:382-387. 1994. Copyright 2002 Ame,~ ican Association for the Advancement of Science. 51)

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Pseudomonas Infections in the NICU

tract infections, and bloodstream infections/ sepsis. Occasionally, it is associated with ventriculoperitoneal shunt infections and endopthalmitis. A unique entity in the neonate is the noma. First described in 1978, 44 n o m a neonatorum is distinctly rare in developed countries with only 1 reported case born in the United States. 45 It is a gangrenous process of the nose, oral cavity, and perineum with a high mortality rate. P aeruginosa is invariably isolated from blood or a culture of the skin lesions.

Treatment

Standard therapy for early onset sepsis o f infants in the NICU is ampicillin/gentamicin or, less desirably, ampicillin/cefotaxime due to the risk of cefotaxime resistance with expanded use of this agent. Vancomycin is usually substituted for ampicillin in the treatment of late onset sepsis. These combinations cover the standard neonatal pathogens; however, it may be p r u d e n t to empirically broaden coverage p e n d i n g antimicrobial sensitivity testing if a gram-negative bacillus is isolated especially from the blood or tracheal aspirate of an infant with an infiltrate on chest radiography (Table 1). In these clinical settings, many experts recomm e n d 2 agents with activity against gram-negative bacilli if the patient is relatively immunosuppressed, such as a VLBW infant, because the consequences of sepsis and p n e u m o n i a are rapid and devastating. 4~ Empiric therapy with ceftazidime or piperacillin/tazobactam or ticarcillin/clavulanate plus an aminoglycoside is acceptable. Antibiotics can then be tailored to the resistance profiles of the isolated organism and the clinical status of the patient. If the resistance

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pattern dictates, a flottroquinolone or a carbap e n e m such as m e r o p e n e m should be used. While animal models of flouroquinolone use have shown cartilage damage, observational studies in large cohorts of cystic fibrosis patients have failed to demonstrate this toxicity even with prolonged or repeated nse. 47,4~ This does not place this class of agents in the category of first line drugs for neonates, but quinolones can be used with relative safety when the situation demands. Urinary tract infections with Pseudomonas in the absence of sepsis can reasonably be treated with a single agent such as ceftazidime or an aminoglycoside if the organism is found to be susceptible? ~ While the use o f 2 agents with activiD' against Pseudomonas has not been studied extensively for central nervous system infections, one could reasonably use ceftazidime plus an aminoglycoside until the infant stabilizes, especiallv if bacteremia is also present. Superficial conjunctival infections can be treated with topical therapy either concentrated aminoglycosides or quinolones; however, deep-seeded ophthalmic infections such as endophthahnitis require systemic therapy usually with a single active agent capable of penetrating the xitreous (quinolone or carbapenem). These infections may also require surgical intervention and intravitreal instillation of antimicrobials.

Conclusion

P aeru~nosa is a well-established hospital-acquired pathogen with the ability, to survive in a variety of environmental conditions. Coupled with broad antimicrobial resistance genes, these factors ensure the long-term survival of this organism in the hospital setting. Control of out-

Table 1. Choices for Empiric Antipseudomonal Therapy by Site of Inl~'ction

Site Bloodstream Tracheal aspirate/pneumonia Eye Urinary tract Central nel~'OUS system

Empi6c Therapy Ceftazidime or pipericillin/tazobactam or ticarcillin/clavulanate + aminoglycoside* Same as for bloodstream Supe,ficial-tobramycin or ciprofloxacin ophthalmic drops Deep-quinolone or meropenem + intravitreal antibiotics Quinolone or an aminoglycoside Ceftazidime or meropenem + / - an anainoglycoside

* Once results of sensitiviLvanalysis are known, antibiotic therapy should be swilched to the agent with the narrowest spectrum of activity so as to help prevent developmen! of antibiotic resistant tlnra. (;ombination therapy should continue until the

patient has stabilizert.

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breaks requires aggressive case finding a n d surveillance of environmental as well as potential staff reservoirs. Traditional epidemiologic control measures such as cohorting of patients a n d contact isolation should be employed immediately u p o n case isolation. T r e a t m e n t should be tailored to the site of infection and may include a beta lactam drug plus an aminoglycoside until the patient stabilizes and the resistance profile of the organism is known. T h e ability of this organism to c o m m u n i c a t e and to coordinate activity with organisms of the same and different species via q u o r u m sensing and biofilm production will continue to challenge our best efforts at t r e a t m e n t eradication. 5~ This may be particularly i m p o r t a n t with infants requiring p r o l o n g e d use of invasive, life-saving devices such as central venous catheters.

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Pseudomonas Infections in the NICU

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