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Mark B. Abelson, MD, Boston, and Lisa Smith, Padova, Italy |
Infectious disease remains a large percentage of an ophthalmologist’s daily patient care. Bacterial conjunctivitis, keratitis, keratoconjunctivitis, blepharitis and contact-lens-related corneal infections are the mainstays of general ophthalmology. Most practices have also encountered endophthalmitis, a much more grave condition.
In the vast majority of cases, selection of the appropriate topical antibiotic usually assures rapid resolution of the infection, but a vigilant eye must be alert to the possibility of resistance to the most frequently used antibiotics. The lag time from diagnosis of the infectious process to bacterial identification necessitates starting therapy with a broad-spectrum antibiotic before classification of the actual cause.
This jumping the gun, while unavoidable for preventing complications, is certainly also to blame for the rising antibiotic resistance observed with all drugs and pathogens. The pharmaceutical industry has attempted to keep abreast with the pace of antibiotic resistance emergence through the development of new drugs and new drug classes. This article will discuss the pertinence of antibiotic resistance to your clinical practice.
Antibiotics and Resistance
While there are more than 24 classes or types of ocular antibiotics available, these large families can be grouped together for the discussion of resistance, since they share common mechanisms of action: The penicillins, aminoglycosides, glycopeptides and fluoroquinolones are the most discussed.1,2
Resistance to an antibiotic depends on the original activity of the antibiotic and the sophistication of the bacteria’s evolving defense system. It can involve quite different phenomena.
• Penicillins. Penicillins, cephalosporins and related ß-lactam antibiotics (monobactams or carbapenems) function by interfering with the biosynthesis of bacterial cell walls, eventually leading to cell lysis and death.
Recent studies have shown that bacteria contain ß-lactam sensitive targets, enzymes known collectively as penicillin-binding proteins. PBPs are found in all bacteria, and microorganisms differ in the amount and type of PBPs present. Interaction of ß-lactam with these proteins inhibits or disrupts cell growth and integrity. Rapid death and lysis only occur due to the activity of endogenous bacterial peptidoglycan hydrolases (autolysins) that are thought to also function in cell division, however. ß-lactams are bactericidal to organisms that contain autolysins and bacteriostatic to those that do not.
Bacteria may be resistant intrinsically to penicillin and other ß-lactam antibiotics, or they may acquire resistance during
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| Bacterial corneal ulcer. Note the hypopyon located in the anterior chamber. | treatment. Resistance can be encoded in chromosomes or plasmids, and the latter are then transferred between bacteria by conjugation. Resistance can result from several mechanisms: alterations of PBPs; secretion of ß-lactamases; and alteration in cell wall permeability to ß-lactams via porins (See Graph below).
Certain bacteria may be resistant due to the absence of PBPs or differences in their structure. These are genetically mediated and may be intrinsic or acquired due to mutation. Altered PBPs with a diminished affinity for penicillins account for the resistance of Staphylococcus aureus to semi-synthetic penicillins (known as “methicillin-resistant” S. aureus or MRSA) and the resistance of Streptococcus pneumoniae to penicillin.
• Aminoglycosides. Aminoglycosides, such as gentamicin and tobramycin, inhibit bacterial ribosomal protein synthesis by binding to the 30S ribosome, exerting a bactericidal effect by interaction with the bacterial cell membrane to cause cell lysis.
Resistance to these antibiotics is mediated by the induction of inhibitory enzymes that render these drugs ineffective. Tetracyclines are only bacteriostatic, blocking protein synthesis by binding to 30S ribosomes, as do the aminoglycosides. Organisms often acquire resistance to tetracyclines by transmission of plasmids, as has occurred with S. aureus, whose resistance to tetracyclines has risen 40 percent in the United States in recent years. In vitro susceptibility testing is critical before selection of a tetracycline as the primary antibiotic therapy.
• Glycopeptides. Glycopeptides such as vancomycin inhibit cell-wall synthesis, preventing peptidoglycan polymerization. This occurs at the second step of cell-wall synthesis as opposed to the third step affected by ß-lactam antibiotics. As such, there is no cross-resistance between these two classes of agents. Vancomycin was thought to be immune to attempts of resistance by bacteria. After more than 40 years, however, cases are now being increasingly documented.
• Carboxyquinolones. Since 1990, a number of fluorinated carboxyquinolones, including ofloxacin, ciprofloxacin and levofloxacin, have
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| A perforated bacterial ulcer caused by pseudomonas. | attained U.S. approval for the treatment of a variety of ocular infectious diseases. The fluoroquinolones exhibit bactericidal activity mainly through interference with the activity of bacterial DNA gyrase. DNA gyrase is a four-subunit bacterial enzyme that plays a vital role in the supercoiling of DNA for storage and for replication, transcription, repair and recombination. Bacteria have also proven adept at developing resistance to quinolones, possibly incurred by the alteration of DNA gyrase or the modification of porins in the cell wall of the bacteria.
The Scope of the Problem
A brief review of reports in the last decade of antibiotic resistance to ocular pathogens puts the scope of this problem in perspective.
Gentamicin and tobramycin resistance is known to be increasing annually in most populations in which they are the drugs of choice for Pseudomonas, Serratia and Staphylococcus infections. The once extremely penicillin-sensitive Streptococcus pneumoniae was found resistant in cases of bacterial keratitis.3 Methicillin-resistant strains of Staphylococcus aureus and Staphylococcus epidermis have increased worldwide, and are recognized as corneal and intraocular pathogens responsible for postoperative endophthalmitis.
In 1998, researchers performed a broad testing of antibiotic susceptibility on 1,291 ocular bacterial isolates from North and South America. This study showed that, while the fluoroquinolones were the most effective against gram-positive and gram-negative organisms combined, 18 percent of staphylococci isolated were resistant to these drugs, and very resistant to other antibiotics (gentamicin, tobramycin and erythromycin).4
Streptococcus pneumoniae is a leading pathogen for the conjunctiva and uveal tract. Researchers identified penicillin-resistant strains of this organism, a leading cause of morbidity in the United States, in ocular and periocular infections from 1975 to 1995. They recommended the cephalosporins and fluoroquinolones as alternatives.5 An Indian study revealed penicillin-resistant strains of Streptococcus pneumoniae in 16 percent of 617 ophthalmic specimens taken over a 14-month period.6
A 1999 study identified the antibiotic susceptibility of coagulase-negative staphylococci isolated from chronic blepharitis, purulent conjunctivitis and suppurative keratitis. Results showed that susceptibility to common antibiotics was variable and unpredictable, and multi-resistant strains were common, leading the authors to suggest antibiotic susceptibility testing in all cases of clinically significant ocular infections caused by coagulase-negative staphylococci.7
Japanese researchers evaluated the growth of methicillin-resistant staphylococci and ofloxacin-resistant bacteria in clinically healthy conjunctivas. Of 194 preoperative eyes, 159 had positive bacterial growth with no clinical signs or symptoms. Of these, only two (one Staphylococcus aureus, one coagulase-negative staphylococcus) were resistant to methicillin, while eight were resistant to ofloxacin (6.7 percent).8
Thus, these resistant bacteria live in the conjunctiva even without signs of infection. On the opposite end of the scale, methicillin-resistant staphylococci are also responsible for a great percentage of staphylococcal-mediated endophthalmitis, with a particular rise in cases of this serious infection without prior hospital or surgical experience.9
The Pediatric Threat
The pediatric population contributes greatly to the patient pool treated with antibiotics and threatened by increasing resistance. A recent study sought to determine the current level of resistance in Haemophilus influenzae and Streptococcus pneumoniae, the primary pathogens of pediatric conjunctivitis. Over a 14-month period in rural Kentucky, researchers prospectively cultured acute conjunctivitis in 250 ambulatory pediatric patients between the ages of 2 and 48 months (average age 24.3 months). They isolated H. influenzae in 42 percent of cases and S. pneumoniae in 30 percent. They detected ß-lactamase in 69 percent of H. influenzae strains and inferred penicillin resistance. Conversely, S. pneumoniae showed less resistance, with 68 percent susceptible, 26 percent resistant and 6 percent intermediate. Conjunctivitis associated with acute otitis media was due to H. influenzae in 57 percent of cases. Ciprofloxacin, ofloxacin and tetracycline were the most active for in vitro antibiotic susceptibility, while gentamicin, tobramycin, polymyxin ß-trimethoprim and polymyxin ß-neomycin were only intermediately active. Sulfamethoxazole possessed no activity against either pathogen.10
H. influenzae still accounts for the great majority of pediatric conjunctivitis or conjunctivitis-otitis syndrome. Researchers reported ß-lactamase production to be only 16 percent of strains of H. influenzae in the early 1980s, rising to 44 percent in the late 1980s, and up to 69 percent in a 2000 report.9 Thus, an alarming rise in resistance has occurred in one of the most frequently seen pediatric pathogens.
A retrospective London study attempted to answer the recurrent question: Has antibiotic sensitivity changed in bacterial ocular infections in this region over time? Researchers identified cases of keratitis from 1984 to
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| Penicillin-binding proteins are found in all bacteria, and microorganisms differ in the amount and type of PBPs present. Interaction of ß-lactam with these proteins inhibits or disrupts cell growth and integrity. |
1999 and culled sensitivity information from disc diffusion technique data. They identified the number of isolates, changes in the proportion of bacterial types, and the number of bacteria fully resistant to ofloxacin monotherapy, dual therapy (gentamicin and cefuroxime) and prophylactic therapy (chloramphenicol).
Of 1,312 bacteria isolated in 13 years, gram-positive bacteria accounted for 54.7 percent of isolates, with Staphylococcal species having been the most prominent (33.4 percent). There was an increase noted in Pseudomonas species, but no overall increase in gram-negative organisms. There was no increase in isolates resistant to ofloxacin since 1995, as well as no increase in dual therapy resistance to gentamicin and cefuroxime. The lack of rising ofloxacin resistance noted in this study may have been due to the investigators’ rigid criteria for proving “complete” resistance. Nevertheless, this study did show a significant increase in gram-negative organisms with chloramphenicol resistance over time.11
Swiss researchers identified the clinical and microbiological profile of bacterial keratitis in one hospital in 85 consecutive, prospectively enrolled patients over a 20-month period. The most commonly isolated bacteria were Staphylococcus epidermis, 40 percent; Staphylococcus aureus, 22 percent; Streptococcus pneumoniae, 8 percent; Pseudomonas, 9 percent; and Moraxella and Serratia, 5 percent. Although fluoroquinolones proved the most efficacious, 1-15 percent of strains were found resistant to this class of antibiotics, as well as 13-22 percent to aminoglycosides, 37 percent to cefazolin, 18 percent to chloramphenicol, 54 percent to polymyxin B, 51 percent to fusidic acid, and 45 percent to bacitracin.12
Fluoroquinolone Resistance
Inevitably, the emergence of resistance to older fluoroquinolones began to be reported. A five-year review has proved that S. aureus is showing increasing resistance to this class (from 5 to 35 percent in five years), with gaps in fluoroquinolone coverage also arising for Streptococcus and coagulase-negative Staphylococcus species.13
This raises concern about monotherapy with these much-prescribed drugs.
Investigators also identified similar trends of fluoroquinolone resistance in bacterial keratitis isolates over a nine-year period in South Florida. They identified the in vitro minimum inhibitory concentrations (MICs: the primary laboratory method used to determine antibiotic resistance) of the corneal isolates to ofloxacin and ciprofloxacin and to the aminoglycosides, gentamicin and tobramycin. During this nine-year period, from 2,920 consecutive corneal cultures, 1,468 (50 percent) recovered a bacterial pathogen. The investigators documented a gradual increase in the number of S. aureus isolates, along with a decrease of P. aeruginosa isolates, as well as an associated decrease in contact lens-associated keratitis and P. aeruginosa infection. Serratia marcescens and P. aeruginosa were still the most common pathogens associated with contact lens infectious complications, however. Most importantly, keratitis-isolated S. aureus was increasingly resistant over time: from 11- to 28-percent resistance to older fluoroquinolones in nine years.
Interestingly, aminoglycoside resistance remained significant but unchanged over time, suggesting a flattening-out phenomenon, while fluoroquinolone usage is rising.14 Decreased effectiveness of older fluoroquinolones to S. aureus will certainly create challenges in ophthalmic practices in the coming years.
Another study identified the overall resistance to ciprofloxacin of pathogens responsible for bacterial keratitis to be 30.7 percent: 32.5 percent of gram-positive cocci; 10 percent of gram-positive bacilli; 13.3 percent of the gram-negative organisms; and 35.1 percent of the actinomyces and related organisms were not sensitive to ciprofloxacin. Cefazolin had the greatest secondary coverage for the gram-positive cocci and actinomyces not sensitive to ciprofloxacin, while gentamicin had the greatest coverage of gram-negative organisms not susceptible to ciprofloxacin. Furthermore, Corneybacterium resistance to ciprofloxacin was found to increase by 10 percent every year, and P. aeruginosa by 3.77 percent every year.15
A 2001 study determined the in vitro susceptibilities of bacterial ocular isolates to the older fluoroquinolones, ciprofloxacin and ofloxacin, in comparison to a new fluoroquinolone, levofloxacin, the L-isomer racemate of ofloxacin. Investigators sent 884 conjunctival specimens of 359 patients with bacterial conjunctivitis to a laboratory for disc diffusion susceptibility testing and the more sensitive broth dilution method.
In 230 bacteria-positive samples (129 gram-positive and 101 gram-negative), they tested 70 distinct species for sensitivity to levofloxacin, ofloxacin and ciprofloxacin. As expected, the new fluoroquinolone won hands down for lack of resistance, with 99 percent and 98 percent susceptibility to gram-negative and gram-positive isolates, respectively, by both disc
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| Staphylococcus corneal ulcer. Gentamicin and tobramycin resistance is known to be increasing annually in most populations for Staphylococcus infections. | diffusion and broth dilution testing. The other fluoroquinolones were also very effective against gram-negative organisms, however, with 96 percent susceptible to ofloxacin and 94-95 percent to ciprofloxacin.
While levofloxacin showed consistent results with the two laboratory methods, the older quinolones diverged for gram-positive bacterial resistance, showing greater susceptibilities with the broth dilution than the disk diffusion technique. They found susceptibility to be 78 percent by disk diffusion and 92 percent by broth dilution for ofloxacin, and 61 percent by disk diffusion and 82 percent by broth dilution for ciprofloxacin. Thus, one can still conclude, particularly for gram-positive species, that levofloxacin was the superior antibiotic that showed the least resistance.16
Most ophthalmologists are still convinced of the vital importance of prophylactic use of antibiotics during and after cataract surgery for the prevention of endophthalmitis. Yet this practice has also certainly contributed considerably to the worldwide problem of rising antibiotic resistance. Most experts reviewing this issue discourage the use of strong and new antibiotics such as vancomycin for this indication, warning that such a practice might lead to the emergence of vision-threatening “super-bugs.”17,18
A Future without Resistance?
A recent study demonstrated the capacity of the enzyme lysostaphin to treat methicillin-resistant Staphylococcus aureus endophthalmitis in the rabbit. Lysostaphin, a zinc metalloproteinase extracted from Staphylococcus simulans, is an enzyme capable of rapidly killing S. aureus by digesting the cell wall. Experimental use of this substance as a therapeutic agent has shown it to be effective and free of side effects for the treatment of bacterial keratitis in the rabbit and in limited systemic use in humans.19
The use of bacteriophages has a completely novel rationale: Fight fire with fire. These are a class of viruses that infect and destroy bacteria and are used exclusively for antibiotic resistant bacteria such as S. aureus. Bacteriophages work by attaching themselves to the host bacterial cell wall, and injecting the phage’s DNA inside. The DNA then quickly replicates, overwhelming and killing the bacteria in the process. Defensive bacterial mutations do not affect their lethality. One such bacteriophage is under investigation for the use in staphylococcus-resistant ocular infections.20
Dr. Abelson is senior clinical scientist at Schepens Eye Research Institute, consults in ophthalmic pharmaceuticals. Ms. Smith is a contract professor at the Department of Medicine, University of Padova, Padova, Italy.
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