Pseudomonas Aeruginosa: Virulence and Pathogenesis Issues
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Published: Fri, 08 Jun 2018
Pseudomonas aeruginosa is a gram negative, motile, aerobic rod shaped bacterium which can occur as singles, in pairs and occasionally in short chains. P.aeruginosa is a ubiquitous organism which can proliferate under the sparest conditions such as sinks, toilets, cosmetics, vaporisers, inhalers, respirators, and anaesthesiology and dialysis equipment. Infected patients and staff are also potential primary sources of infection (2). P.aeruginosa is a major opportunistic pathogen of the immunocompromised causing a wide range of nosocomial infections. These include infections of burn, post operative wounds, urinary tract (especially in patients with catheters), ears and eyes. Infection frequently leads to sepsis and deaths can occur (3). This organism is associated with the greatest morbidity and mortality in cystic fibrosis (4) and is prevalent among patients with burn wounds and intravenous drug users (5, 6). P.aeruginosa is able to persist and multiply in moist environments and on most pieces of equipment in hospital wards. This is of importance in cross infection control (4).
P.aeruginosa is the most important, resistant and dangerous organism infecting burn patients (7). It is the fifth common pathogen among hospital microorganisms and causes 10% of all hospital acquired infections (8). The rate of commensalisation increases as the duration of hospital stay increases (9). Epidermiologically, P.aeruginosa is ranked as the fourth cause of nosocomial infections in the United States (10). A study on various clinical isolates was conducted in Afghanistan at the Post Graduate Medical Institute (PGMI) Hayatabal Medical Complex to ascertain the prevalence and antimicrobial susceptibility patterns of P.aeruginosa infections. Among the positive isolates, 6.67% were P.aeruginosa with the highest rate of infection observed in orthopaedic ward (24.61%) and 0PD (20%). The highest percentage of P.aeruginosa isolates were observed in pus (57.64%) (11).
Pseudomonas aeruginosa is resistant to many antimicrobial agents and has therefore become dominant and important when the more susceptible bacteria of the normal flora are suppressed (1). With the widespread use of quinolones both in the hospital and in the common setting, drug resistant P.aeruginosa isolates have emerged and continue to escalate rapidly (12). The antimicrobial agents are losing their efficacy due to indiscriminate use of antibiotics, lack of awareness, patient non compliance and unhygienic conditions (11). Like most gram negative bacilli, P.aeruginosa has been reported to have developed resistance to commonly used antibiotics and disinfectants. It was originally sensitive to Carbernicillin, piperacillin, Gentamicin, ciprofloxacin and other drugs. However it is now resistant to these antibiotics (13, 14). The virulence factors associated with P.aeruginosa infections include cytotoxin production, the organism’s ability to form a biofilm, produce gelatinase, elastase and alkaline protease. These cause the destruction of connective tissue and degradation of host immunological factors (15, 16). The primary aim of this study is to determine the prevalence of P.aeruginosa in wound infections and its sensitivity to commonly used antibiotics in inpatients at Parirenyatwa Group of Hospitals.
Pseudomonas aeruginosa is one of the most common Gram-negative microorganisms identified in the clinical specimens of hospital admitted patients. It is a rod that measures about 0.6 – 2µm and is motile by means of a single polar flagellum (1, 18).
P. aeruginosa is noted for its metabolic versatility and its exceptional ability to colonize a wide variety of environments and also for its intrinsic resistance to a wide variety of antimicrobial agents. It is an obligate aerobe that grows well at temperatures between 37-42°C. Due to its ubiquitous nature, P. aeruginosa grows readily on any type of media (1). On Blood agar they are often β-haemolytic while on MacConkey agar they produce pale colonies because they do not ferment lactose. P.aeruginosa is oxidase positive (17).
Pseudomonas aeruginosa is well known for its production of two soluble pigments, pyocyanin which is a non-fluorescent bluish pigment and the fluorescent pigment pyoverdin, which gives a greenish colour to the media. P. aeruginosa also produces a sweet grape-like odour due to the production of 2-aminoacetophenone (1, 17).
P. aeruginosa is pathogenic when introduced to areas lacking normal host defences for example when there is tissue damage and during cancer therapy where there is neutropaenia (1). P.aeruginosa is a major opportunistic pathogen of the immunocompromised causing a wide range of nosocomial infections. These include infections of burn, post operative wounds, urinary tract (especially in patients with catheters), ears and eyes (in users of extended-wear soft contact lenses). Infection frequently leads to sepsis and deaths can occur (3). This organism is associated with the greatest morbidity and mortality in cystic fibrosis (4) and is prevalent among patients with burn wounds and intravenous drug users (5, 6).
It produces cytotoxins, proteases and haemolysins. Isolates from patients with Cystic fibrosis produce a polysaccharide, alginate. The alginate, pili and outer membrane mediate adhesion to host epithelia (19).
The ability of Pseudomonas aeruginosa to cause a wide range of infections is due to its ability to produce a number of cell-associated (adhesions, alginate, pili, flagella and lipopolysaccharide) and extracellular (elastase, exoenzyme S, exotoxin A, haemolysins, iron binding proteins, leukocidins and proteases) virulence factors. These mediate a number of processes including adhesion, nutrient acquisition, immune system evasion, leukocyte killing, tissue adhesion and blood stream invasion (20, 21).
CELL-ASSOCIATED VIRULENCE FACTORS
P.aeruginosa requires a breach in first-line defences to initiate infection. This can result from alteration of the immunologic defence mechanisms for example in chemotherapy-induced immunosuppression and AIDS, disruption of the protective balance of mucosal normal flora by broad-spectrum antibiotics, or breach of normal mucosal barriers for example trauma and burns (21, 23).
Adherence of P. aeruginosa to host epithelium is mediated by type 4 pili, that extend from the cell surface (1, 22). Flagella, primarily responsible for motility may also act as adhesins to epithelial cells (23). Lipopolysaccharides are responsible for endotoxic properties of the organism while the exopolysacharride is responsible for the mucoid colonies from patients with Cystic fibrosis (1).
EXTRACELLULAR VIRULENCE FACTORS
These are extracellular products produced by P. aeruginosa that can cause extensive tissue damage. They include exotoxin A, exoenzyme S, elastase, alkaline protease but the contribution of a given factor varies with the type of infection (24).
Exotoxin A catalyses ADP-ribosylation and inactivation of elongation factor 2, leading to inhibition of protein biosynthesis and cell death (25). It is also responsible for local tissue necrosis (1). Exoenzyme S is also an ADP-riboslytransferase that ribosylates GTP binding proteins resulting in direct tissue damage (26). Phospholipase C and rhamnolipid are haemolysin produced by P. aeruginosa. They breakdown lipids and lecithin and both have cytotoxic effects (27). Pseudomonas aeruginosa also produces toxins which include Las B elastase, Las A elastase and alkaline protease (28). Las A elastase and Las B elastase have elastolytic activity. Elastin is a major component of lung tissue and blood vessels. Las B elastase is a zinc metalloprotease while Las A is a protease. Alkaline protease lyses fibrin (29).
Pseudomonas aeruginosa is also able to form biofilms. Biofilms are complex communities of surface-attached aggregates of microorganisms embedded in a self-secreted extracellular polysaccharide matrix or slime (alginate) (30, 31). These act as efficient barriers against antimicrobial agents (aminoglycosides, β lactamases, fluoroqunilones and disinfectants) and the host immune system resulting in persistent colonisation and loss of action at the site of infection (32, 33).
CELL TO CELL SIGNALLING
Cell to cell signalling systems control extracellular virulence factors required for tissue invasion by P. aeruginosa.
THE LAS CELL TO CELL SIGNALLING SYSTEM
The Las cell to cell signalling system regulates the expression of Las B elastase (34). It regulates Las B expression and is required for optimal production of other extracellular virulence factors such as Las A elastase and exotoxin A (35).
THE RHL CELL TO CELL SIGNALLING SYSTEM
The rhl cell to cell signalling system controls the production of rhamnolipid. The system regulates the expression of the rhl AB operon that encodes a rhamnosyltransferase required for rhamnolipid production. It is also important for Las B elastase, protease, pyocyanin and alkaline transferase production (36).
ANTIMICROBIAL REACTIVITY OF P. AERUGINOSA
Like most gram negative bacilli, P.aeruginosa has been reported to have developed resistance to commonly used antibiotics and disinfectants. It was originally sensitive to Carbernicillin, piperacillin, Gentamicin, ciprofloxacin and other drugs. Degrees of cross-resistance between these agents have been reported however (13, 14). Treatment of infections by P. aeruginosa is often difficult because of its virulence and limited choice of antimicrobial agents. P. aeruginosa has the capacity to carry multiresistance plasmids, and this feature has led to the appearance of some strains that are resistant to all reliable antibiotics (37). In a study carried out at the Post Graduate Medical Institute Hayatabad Medical complex in Afghanistan on the prevalence and resistance pattern of P. aeruginosa against various antibiotics, the highest resistance was observed against ampicillin, ampicillin/ sulbactam, co-amoxiclave and ofloxacin and least resistance was observed against amikacin. Similarly the MIC for ampicillin, ampicillin/sulbactam and co-amoxiclave against clinical isolates of Pseudomonas aeruginosa was also high (11). A similar study carried out at Dhaka Medical College Hospital in 2006 showed that almost all of the P. aeruginosa isolates were resistant to cefixime and co-trimoxazole, majority were resistant to ceftazidime, gentamycin and ciprofloxacin. The result of the study showed that imipenem is the most effective drug against P. aeruginosa, followed by amikacin and ciprofloxacin (39).
MECHANISM OF ACTION OF COMMONLY USED ANTIBIOTICS
The commonly used antibiotics in the treatment of P. aeruginosa infections are Aminoglycosides (for example Gentamicin and amikacin), Penicillins (such as cabernicillin), Quinolones (for example Nalidixic acid, ciprofloxacin and levofloxacin), Cephalosporins (ceftazidime) and Carbapenemes (meropenem and imipenem) (37).
Penicillins, Cephalosporins and Carbapenemes inhibit bacterial cell wall synthesis. They are β-lactam agents. Aminoglycosides and Tetracyclines are inhibitors of protein synthesis. Quinolones are inhibitors of bacterial nucleic acid synthesis (17).
RESISTANCE TO ANTIBIOTICS
The antimicrobial resistance conferred by P. aeruginosa is due to mutations in the organism’s genetic material. No single mutation is responsible for multidrug resistance. Mutations to topoisomerase 2 and 4 confer fluoroquinolone resistance. Derepression of the chromosomal AmpC β-lactamase reduces susceptibility to penicillins and cephalosporins. Up-regulation of MexAB-OprM compromises the fluoroquinolones, penicillins, cephalosporins and it also enhances resistance to many other drugs that lack useful anti-pseudomonal actions (38).
1.2 STATEMENT OF THE PROBLEM
Considering the ability of Pseudomonas aeruginosa to persist and multiply in moist places and in most pieces of equipment in hospital wards (4), antimicrobial resistance is a growing concern. This is attributed to the fact that the organism is able to withstand conditions such as high temperature and high concentrations of salts and antiseptic (10). It is therefore imperative to constantly evaluate the pathogenesis and sensitivity patterns of P.aeruginosa so as to prevent further spread and recurrence of infection in the hospital set up.
Null Hypothesis (H0)
The prevalence of P. aeruginosa in wound and pus swab specimens at Parirenyatwa hospital is 6.7%.
Alternative Hypothesis (H1)
The prevalence of P. aeruginosa in wound and pus swab specimens at Parirenyatwa hospital is greater than 6.7%.
The aims of this study are:
- To determine the prevalence of P. aeruginosa in wound and pus swab specimens of patients admitted at Parirenyatwa hospital.
- To determine the antimicrobial susceptibility patterns of the isolates.
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