Management of Psuedomonas Keratitis: a review article

Bacterial keratitis can lead to severe vision loss and corneal scarring, and possibly perforation. Early and appropriate management is a key factor in decreasing and preventing complication. Pubmed and Medline were searched for articles related to Pseudomonas keratitis between year 2000 and 2017 to get current guidelines about the management of Pseudomonas keratitis. These articles are reviewed in this article and information related to management is summarized. The most used agents to treat Pseudomonas are either aminoglycosides (usually gentamicin) fortified with a cephalosporin or mono therapy with a fluoroquinolones usually ciprofloxacin. In most areas, most strains of Pseudomonas were susceptible to ciprofloxacin. The role of topical steroids is discussed, as well as, available options for treatment of multidrug resistant Pseudomonas species. Management of Psuedomonas keratitis: A review article Mouna Mohamad Al Saad1 1 Faculty of Medicine, Department of special surgery/Division Ophthalmology. The University of Jordan, Amman, Jordan. Contact information: Dr. Mouna Mohamad Al Saad.  m.alsaad@ju.edu.jo.


Introduction
Bacterial keratitis is a vision threating infection characterized by epithelium loss, stromal infiltration by white blood cells and melting of the corneal stoma. It happens when one of the protective mechanisms of the ocular surface is sacrificed [1].
Early recognition and administration of appropriate choice of antibiotics is crucial if vision is to be restored. Pseudomonas species is commonly associated with contact lens wear [1][2][3][4][5]. Predisposing risk factors depend on the geographical locations and can be influenced by the penetration of contact lens. Trauma to the eye is an important risk factor in developing countries, meanwhile contact lens wear remains the main cause in developed countries [6]. A study from Malaysia suggested that Pseudomonas spp. is the main microorganism following vegetation related corneal injury is some regions. Pseudomonas group is a common inhabitant of soil, water and vegetation [7].
Other risk factors for bacterial keratitis include disease of the ocular surface and previous ocular surgeries [8].
Seasonal variation was noted in the presentation of Pseudomonas spp. keratitis, being mostly frequent in summer. Possibly warmer temperature, higher humidity and greater exposure to water may explain the higher incidence in summer [9].

Biological characterization of Pseudomonas species
Pseudomonas species is widespread in the environment and it can be isolated from various living sources, including plants, animals, and humans [10]. The organism is non-fermenting gram-negative, non-spore-forming, strictly aerobic, and catalase and oxidase positive. It can survive at temperature between 4 to 42ºC but not at pH values below 4-5. Pseudomonas has also the ability to survive on minimal nutritional requirements and to tolerate a variety of physical conditions which allow it to persist in both community and hospital settings [10]. The organism forms biofilms on wet surfaces and in hospital environment such as sinks, humidifiers, and respiratory therapy equipment [11]. There are numerous Pseudomona s spp. which have been associated with opportunistic infections include P. aeruginosa, Pseudomonas maltophilia, P. fluorescens, P.putida, P. cepacia, P. stutzeri, and P. putrefaciens, but P. aeruginosa is the most common reported species causing human infection [12].

Virulence of P. aeruginosa
Pseudomonas aeruginosa is a clinically important and opportunistic pathogen, often causing nosocomial infections. P. aeruginosa produces more than other species a variety of virulence factors, whose expression is regulated by different regulatory systems especially by Quorum sensing (QS) system. Most clinical isolated strains of P. aeruginosa can produce many virulence factors such as elastase, alginate, exotoxin A, exotoxin S, exotoxin U, alkaline protease and pyocyanin [13,14]. P. aeruginosa has a single polar flagellum that is responsible for motility. It is also involved in the adhesion to respiratory epithelial cells. This virulence effect induces an inflammatory response by interaction with receptor cells. P. aeruginosa synthesizes the exopolysaccharide alginate in response to environmental conditions. Alginate serves to protect the bacteria as biofilms in its surrounding environment and also enhances adhesion to solid surfaces. The organism can produce an alginate enzyme under certain infection conditions and both alginate biosynthetic and degradative enzymes are important for the development, maintenance and spread of P. aeruginosa biofilms in infected patients [14,15].
Antimicrobial resistance of P. aeruginosa P. aeruginosa is the most common species and it colonizes more than 50% of hospitalized patients. P. aeruginosa is a leading cause of nosocomial infections, ranking second among the gram-negative pathogens reported to the National Nosocomial Infection Surveillance System in USA. The organism is responsible for severe nosocomial infections such as septicemia and pneumonia, life-threatening infections in immunocompromised persons, and chronic infections in cystic fibrosis patients [16].
A recent systemic review and meta-analysis study indicated that hospitalized patients with Multidrugresistant (MDR) P. aeruginosa infections appear to have increased mortality and hospital length [17,18]. Infections caused by MDR P. aeruginosa are difficult to treat as the majority of isolates exhibit varying degrees of innate resistance or the constitutive expression of AmpC β-lactamase and efflux pumps, combined with low permeability of the outer membrane. Acquired resistance is also reported due to the production of plasmid mediated AmpC beta (β)-lactamase, ESBLs and Metallo β-lactamase (MBL) enzymes [18]. Additionally, P. aeruginosa isolates from environment including water, sewage and human feces indicated the presence of high resistance rates to commonly used drugs in many Arab countries [19][20][21][22][23].

Epidemiology P. aeruginosa keratitis
The occurring incidence rate of Pseudomonas aeruginosa keratitis was constant over the last 20 years at 1/2500 of daily contact wearers and 1/500 of extended wear contact lens [6].
The incidence of Pseudomonas keratitis varies depending on geographical location. Template zones have a higher incidence rate of Gram positive bacteria causing keratitis and less aggressive disease [6]. However, Pseudomonas was found to be the most common Gran negative bacterial isolated in Toronto, Canada over the past 16 years [24] and in Western India with an incidence of 17.14% [1]. In Spain an incidence of 12.5% of all bacterial keratitis was reported and was found to be the most frequent Gram negative microorganisms [8]. In addition, this incidence was found to be 12.5% in China [2]. In Saudi Arabia, an incidence of 38.4% was reported [3].On the other hand, it has been reported that up to 80% of all positive cultures in Iran are caused by Pseudomonas spp. [4]. Moreover, P. aeruginosa was the most frequent bacteria isolated in contact Lens related ulcers with an incidence of 75% in France [5] and 79.7% in Malaysia [7]. A higher percentage of contact lens related cases of Pseudomonas ulcers was found in a five year period from 2009 to 2014 in Texas U.S.A. [25].
Prognosis of bacterial keratitis depends on the distance from the limbus and the minimal inhibitory concentration (MIC) of the first antibiotic used or the lowest MIC if combined treatment is used [6].
The presence of conjunctival chemosis was found to be a clinical clue to the Pseudomonas as a causative agent in bacterial Keratitis [26]. Large ulcers are more likely to be caused by Pseudomonas [6].
The stain type of Pseudomonas aeruginosa (invasive or cytotoxic) may affect the clinical presentation, prognosis and response to antibiotics. It was suggested thatpseudomonas keratitis caused by in-vasive strains had better visual acuity than ones caused by cytotoxic strains. On the other hand, invasive ulcers are less significantly improved compared to cytotoxic ones at three months [27].
Risk factors for multiple and extremely drug resistance Pseudomonas include: bandage contact lens, topical steroids, previous corneal graft and ocular surface disease following Steven Johnson disease [28].

Management of P. aeruginosa keratitis
As a rule, every case of suspected microbial keratitis should be scrapped for culture and sensitivity, especially with the increasing number of microbial resistant strains. Scrapping should be done with a surgical blade such as Bard Parker blade No.15, Kimora spatula or 21 gauge disposable needle. Samples should be sent immediately to bacteriology laboratory. The sample is normally inoculated on chocolate agar, blood agar, thioglycolate broth and incubated at 37ºC for 24-48 hours. Sabouraud agar plates should be also used for detection of yeast and rapid growing fungi. These plates should be kept at room temperature and 37 ºC for at least 4 days. Scrapping of small lesions (less than 2 mm 2 ) is not advised and is not worthwhile. As a result, small lesions should be treated empirically. Scrapes should also be sent for preparing a Gram stains [6].
Clinical evaluation to antimicrobial response include: blurring of the margins to the infiltrates, decreased intensity of the infiltrates, decreased stromal edema and endothelial plaques, reduced anterior chamber reaction, re-epithelialization and cessation of corneal melting.
Topical antibiotics are used widely and empirically. Topical antibiotics remain the best treatment for bacterial keratitis. It was found that all frequently used antibiotics are equally effective, but the final outcome is not satisfactory because of corneal melting, scaring and perforation [29]. Treatment used varies depending on geographical location and available local antimicrobial susceptibility of Pseudomonas isolates. Monotherapy with ciprofloxacin of 0.3% (or other fluoroquinolones) is commonly used. Sub conjunctival injections of gentamicin may be also used. The combined use of two fortified antibiotics preparation of 1.5% gentamicin and 5% cefuroxime covers almost the entire spectrum of common causative bacterial pathogens [30]. Randomized clinical trials have demonstrated that monotherapy with fluoroquinolones is not inferior and has fewer side effects compared with combination treatment [31].
A study from Iran recommended the use of combination of ceftazidime and amikacin or ceftazidime and ciprofloxacin as the first treatment based on antibiotic sensitivity of isolated microbes. In that study it was shown that Pseudomonas isolates were resistant to chloramphenicol, trimethoprim, vancomycin and cefazolin. Moreover, these antibacterial agents should not be used as initial therapy in that region [32]. For example, In Australia, the major ocular pathogens are generally susceptible to the most commonly used antibiotics to treat microbial keratitis, and fluoroquinolones, aminoglycosides and cephalosporins is generally reserved for treatment of significant microbial keratitis [33].
In China, it was shown that most Gram-negative isolates including Pseudomonas was sensitive to neomycin but resistant to chloramphenicol [2]. A study from Iraq revealed that the use of ciprofloxacin alone as a starting treatment does not cover most of the bacteria causing superlative microbial keratitis. Another anti-microbial agent should be added. The choice of which treatment to be used depends on the likely microbial agent, taking into consideration the regional risk factors and clinical characteristics [30].
In Brazil, Pseudomonas isolates were susceptible to ciprofloxacin and ofloxcacin. One out of 239 cases was found to be resistant to gentamicin. Resistance to fourth generation fluoroquinolones was not observed [34]. In a recent study in South India, Pseudomonas isolates were found to be the second most common resistant bacterial isolate with resis-tance in bacterial Keratitis after Streptococcus pneumonia. The resistance was relatively uncommon and did not increase over 12 years (from 2002 to 2013) [35]. Data from Iran suggest that Pseudomonas isolates were mostly sensitive to ciprofloxacin followed by imipenem, meropenem and ceftazidime [4]. In Canada an increasing resistance to erythromycin (p=0.018), ceftazidime (P=0.046), piperacillin/tazobactam (P=0.005) was seen in the past 16 years [21].
In Spain it is recommended to use aminoglycosides (gentamycin) and fluoroquinolones (ciprofloxacin) and avoid erythromycin due to increasing resistance to it [22].
A recent study evaluated the effect of different antibiotics on P. aeruginosa in vitro of primary human corneal fibroblasts. It was shown that the highest activity against planktonic pseudomonas was ciprofloxacin, Levofloxacin, and polymyxin B, followed by gentamicin and ofloxacin. Cefuroxime and chloramphenicol was found to be ineffective. It was also found that bactericidal and bacteriostatic antibiotics used in bacterial keratitis were unable to eradicate Pseudomonas infection of human fibroblasts culture in vitro. Their effectiveness depends on the cellular location of Pseudomonas [36].
According to the susceptibility of Pseudomonas strain the response to topical treatment may be affected. Adjuvant treatment with steroids, when used with moxifloxacin, was associated with better visual outcome in Pseudomonas keratitis caused by invasive strains compared to ones caused by cytotoxic strains. The size of scar at 3 months in the invasive strains was smaller when steroids and moxifloxacin were used compared to moxifloxacin alone. Less improvement was noted in the cytotoxic group (p=0.07) [7,27].
Most ophthalmologists recommend starting anti-microbial empirically before culture results are available [37][38][39]. A study from Japan showed better results when anti-microbial agents were selected based on culture results, stressing the impor-tance of culture results [40]. However, a study from Oxford showed that 93.2% of bacteria isolated from infected corneas including Gram-positive and Gram-negative were sensitive to ciprofloxacin. On the other hand, 99.5% were sensitive to the combination of gentamicin and cefuroxime (P=0.0015). It was reported that there was no increase in the resistance over a decade, as indicated in the study period [41].
In some resistant cases, a combination of piperacillin/tazobactam may be useful [27]. The use of topical steroids (prednisone phosphate) combined with moxifloxacin did not improve the overall clinical outcome as showed in a multi-center clinical study that enrolled patients from India and USA [42].
In general, P. aeruginosa is mostly susceptible to fluoroquinolones, but multi drug resistant Pseudomonas strains are reported increasing in many country. For example, in Austria, strains are resistant to ciprofloxacin, gentamicin, tobramycin and amikacin but susceptible to ceftazidime, imipenem, meropenem and imipenim. It was also found that P. aeruginosa isolates has shown average susceptibility to piperacillin and ticarcillin [42].
Possible synergistic activity is documented between anti-microbial against Pseudomonas. Meropenem and ciprofloxacin with 90% of isolates showed synergistic or additive effect. This made combined anti-microbial a potential treatment against multi drug resistant strains [29]. Most common prescribed drugs are ciprofloxacin and tobramycin, a fortified aminoglycosides. Geographical location should be taken in consideration before giving them as treatment for P. aeruginosa [30].
Multi drug and extreme drug resistant Pseudomonas isolates are very difficult to treat. More than half of those patients required therapeutic grafts. Most sensitivity was reported to imipenem followed by colistin and neomycin. Few patients were sensitive to ceftazidime and azithromycin [25].
In advanced keratitis, continues infusion of topical antibiotics (ceftazidime 50 mg/ ml) with the Morgan lens was effective when applied for one week followed by conventional topical antibiotics [44]. The early use of 19% topical colistin was found to be effective in the treatment of multi drug resistant Pseudomonas with few side effects [42]. Topical colitithemate of 1.6% maybe an effective topical alternative [43]. Corneal cross-linking is a promising new modality for non-healing corneal ulcers including ulcers secondary to Pseudomonas keratitis [47][48]. Amniotic membrane transplantation is effective for relieving pain, decreasing inflammation, stabilizing the cornea and promoting epithelial healing along with topical treatment [49]. It was found that double layered amniotic membrane transplantation had a good long term outcome in Pseudomonas infected corneas with descematocele. It reduced the risk of perforation and the need for emergent corneal grafting [50]. Salicylic acid may be beneficial as an anti-microbial agent in the treatment of Pseudomonas aeruginosa. It was shown that salicylic acid alters the membrane proteome of Pseudomonas aeruginosa, mildly increased the resistance to carbapenem antibiotics only with no effect on other antibiotics. Salicylic acid also reduces the clinical score and the number of bacteria with no effect on the number of neutrophils [51].

Conclusion
Pseudomonas, especially P. aeruginosa is mostly related to contact lens induced microbial keratitis.The most common used therapy is mono therapy with a fluoroquinolone or fortified aminoglycosides. Most strains are still susceptible to these antibiotics but geographical location should be taken in consideration. Multidrug resistant Pseudomonas isolates are observed in clinical practice and different treatment modalities are available.