The In Vitro Evaluation of Tigecycline and the In Vivo Evaluation of RPX-978 (0.5% Tigecycline) as an Ocular Antibiotic

Staphylococcus aureus Keratitis: Incidence, Pathophysiology, Risk Factors and Novel Strategies for Treatment

Bacterial keratitis is a devastating condition that can rapidly progress to serious complications if not treated promptly. Certain causative microorganisms such as Staphylococcus aureus and Pseudomonas aeruginosa are notorious for their resistance to antibiotics. Resistant bacterial keratitis results in poorer outcomes such as scarring and the need for surgical intervention. Thorough understanding of the causative pathogen and its virulence factors is vital for the discovery of novel treatments to avoid further antibiotic resistance. While much has been previously reported on P. aeruginosa, S. aureus has been less extensively studied. This review aims to give a brief overview of S. aureus epidemiology, pathophysiology and clinical characteristics as well as summarise the current evidence for potential novel therapies.

Topical Vancomycin 5% Is More Efficacious Than 2.5% and 1.25% for Reducing Viable Methicillin-Resistant Staphylococcus aureus in Infectious Keratitis

PURPOSE

Topical vancomycin 5% (50 mg/mL) has been used for the treatment of methicillin-resistant Staphylococcus aureus (MRSA) keratitis, but patient comfort has many clinicians using lower concentrations. We compared the efficacy of different concentrations of vancomycin in the treatment of experimental MRSA keratitis.

METHODS

The corneas of 45 rabbits were infected with 2000 colony-forming units (CFUs) of MRSA. Corneal epithelium was abraded in the left eyes to mimic corneal ulceration. After 4 hours, the corneal CFUs were determined at the onset of treatment. The remaining rabbits were divided into 4 treatment groups (n = 9): 1) vancomycin 5%, 2) vancomycin 2.5%, 3) vancomycin 1.25%, and 4) saline. The rabbits were treated topically in both eyes every 15 minutes for 5 hours. One hour after treatment, the rabbits were clinically examined and euthanized, corneas were removed, and CFUs were determined to analyze vancomycin penetration, treatment efficacy, and bactericidal effect.

RESULTS

Ocular toxicity was concentration dependent from mild to moderate. For the abraded corneas, the CFUs of the vancomycin 5% group were lower than 2.5% and 1.25%, and all vancomycin groups were lower than saline. The CFUs of 2.5% were lower but similar to 1.25%. The vancomycin 5% group demonstrated a bactericidal effect and the best penetration. The CFUs of the abraded corneas treated with saline were lower than those of the intact corneas, indicating a possible antibacterial effect from the ocular surface.

CONCLUSIONS

Vancomycin 5% was most potent for treating experimental MRSA keratitis. The clinician may need to reassess treatment regarding antibacterial efficacy and patient comfort.

Minimum inhibitory concentrations and resistance for selected antimicrobial agents (including imipenem, linezolid and tigecycline) of bacteria obtained from eye infections

Objective: To determine bacteria obtained from eye infections, both resistance and minimal inhibitory concentration (MIC) to gatifloxacin, moxifloxacin, tigecycline, linezolid and imipenem, in vitro. Methods: A cross-sectional descriptive study was undergone with 50 samples from 50 eyes of patients diagnosed with keratitis or endophthalmitis, who came to a consultation at the Fundación Oftalmológica de Santander (Floridablanca, Colombia) from August to November 2014. The MICs of the isolated microorganisms were established through Etest® strips (BioMérieux SA, Marcy-l'Etoile - France). Results: Of the 50 samples in total, 17 different bacteria species or groups were isolated. The main isolate for gram-positives was Methicillin Resistant Coagulase-Negative Staphylococcus (17 samples), and for gram-negatives was Pseudomonas aeruginosa (6 samples). The susceptibility percentages sorted from highest to lowest for gram-positive isolates (n=38) were: imipenem 90.3%, linezolid 87.9%, tigecycline 78.1%, gatifloxacin 68.8% and moxifloxacin 68.8%. For gram-negative isolates (n=12), they were: imipenem 72.7%, gatifloxacin 70%, moxifloxacin 66.7% (no reference cut-off points were found for Pseudomonas aeruginosa), tigecycline 22.2%, and linezolid 0% (as expected according to its inhibition spectrum). Conclusions: Although fourth generation fluoroquinolones are currently the preferred initial empirical monotherapy in our practice, given the increasing bacterial resistance, in cases in which gram-positive bacteria were isolated in the initial staining imipenem, linezolid or tigecycline could be used as an alternative. On the other hand, for cases of gram-negative bacteria, no antimicrobial susceptibility exceeded 80%, so using two antimicrobials looking for a synergy between them could be a better option. Abbreviations: S = Susceptibility, IS = Intermediate susceptibility, R = Resistance.

Postsurgical Cataract Prophylaxis With Intravitreal "Triamcinolone-Moxifloxacin" May Not Be Optimal For Preventing Endophthalmitis

PURPOSE

Intravitreal injections of antibiotics and anti-inflammatories are used by some cataract surgeons for surgical prophylaxis. To support this prophylaxis, intravitreal triamcinolone-moxifloxacin (TM) and triamcinolone-moxifloxacin-vancomycin (TMV) were tested for preventing Staphylococcus aureus (SA) endophthalmitis in rabbits.

METHODS

Trademark formulations of TM (15/1 mg/mL) and TMV (15/1/10 mg/mL) were intravitreally injected into seven groups of eight rabbits each (A-G). Before intravitreal injection, the vitreous was first challenged with clinical SA endophthalmitis isolates (5,000 colony-forming unit) with varying minimum inhibitory concentrations (MICs in μg/mL) to moxifloxacin (denoted by the MIC at the end of each group listed): A) TMV-10, B) TM-10, C) Saline-10, D) TM-2, E) Saline-2, F) TM-0.032, and G) Saline-0.032. After 24 hr, the rabbit eyes were graded for clinical endophthalmitis and cultured for viable SA.

RESULTS

Rabbits treated with TMV and challenged by SA with a moxifloxacin MIC of 10 μg/mL did not present with endophthalmitis (0/8, no eyes with endophthalmitis). For SA with moxifloxacin MICs of 10.0 and 2.0 μg/mL, TM did not prevent endophthalmitis (16/16, 100% of eyes with endophthalmitis). For SA with a moxifloxacin MIC of 0.032 μg/mL, endophthalmitis was prevented with TM (0/8, no eyes with endophthalmitis). All saline-treated eyes developed endophthalmitis (23/23, 100% of eyes with endophthalmitis).

CONCLUSIONS

Intravitreal monotherapy with TM did not provide consistent prevention of SA endophthalmitis, whereas intravitreal TMV successfully prevented endophthalmitis because of SA with elevated MIC values to moxifloxacin. Cataract surgeons need to be aware that vancomycin seems to be essential for intravitreal prophylaxis to cover moxifloxacin resistance.

New Is Old, and Old Is New: Recent Advances in Antibiotic-Based, Antibiotic-Free and Ethnomedical Treatments against Methicillin-Resistant Staphylococcus aureus Wound Infections

Staphylococcus aureus is the most common pathogen of wound infections. Thus far, methicillin-resistant S. aureus (MRSA) has become the major causative agent in wound infections, especially for nosocomial infections. MRSA infections are seldom eradicated by routine antimicrobial therapies. More concerning, some strains have become resistant to the newest antibiotics of last resort. Furthermore, horizontal transfer of a polymyxin resistance gene, mcr-1, has been identified in Enterobacteriaceae, by which resistance to the last group of antibiotics will likely spread rapidly. The worst-case scenario, "a return to the pre-antibiotic era", is likely in sight. A perpetual goal for antibiotic research is the discovery of an antibiotic that lacks resistance potential, such as the recent discovery of teixobactin. However, when considering the issue from an ecological and evolutionary standpoint, it is evident that it is insufficient to solve the antibiotic dilemma through the use of antibiotics themselves. In this review, we summarized recent advances in antibiotic-based, antibiotic-free and ethnomedical treatments against MRSA wound infections to identify new clues to solve the antibiotic dilemma. One potential solution is to use ethnomedical drugs topically. Some ethnomedical drugs have been demonstrated to be effective antimicrobials against MRSA. A decline in antibiotic resistance can therefore be expected, as has been demonstrated when antibiotic-free treatments were used to limit the use of antibiotics. It is also anticipated that these drugs will have low resistance potential, although there is only minimal evidence to support this claim to date. More clinical trials and animal tests should be conducted on this topic.