4. Discussion
The susceptibility of bacteria to antibiotics in our study was similar to those reported in the literature but not for all types of antibiotic agents. Gram-positive bacteria responded well to vancomycin in all tested cases, except one were the bacteria had intermediate sensitivity to the antimicrobial. This fact makes it extremely valuable in the treatment of bacterial endophthalmitis administrated through intravitreal injections. Nevertheless, intracameral administration of vancomycin as a prevention strategy for endophthalmitis has been associated with the development of retinal vasculitis [
9,
10,
11]. This prevention treatment is mainly practiced in Australia and less common in European countries [
11]. Vancomycin does not have an antimicrobial effect on Gram-negative bacteria and is usually associated with other antibiotic classes like aminoglycosides or ceftazidime in the treatment of bacterial endophthalmitis [
12]. Adjunctive systemic antibiotics that achieve intravitreal therapeutic levels can also be administered, best known agents being meropenem, moxifloxacin and linezolid [
13]. Very rare gram-positive bacteria like coagulase-negative staphylococcus and enterococcus have been found to express reduced susceptibility to vancomycin in bacterial endophthalmitis [
14,
15,
16]. For such rare cases of vancomycin resistant enterococci, intravitreal administration of linezolid was attempted but concerns regarding the safety and adverse reactions are still raised [
17]. Nevertheless, the association of vancomycin 1.0 mg/0.1 ml together with ceftazidime 2.25 mg/0.1 ml remains the most frequently used therapeutic scheme in the management of bacterial endophthalmitis [
18]. This combination covers a broad-spectrum of microbial agents. In allergic patients to ceftazidime, vancomycin can be associated instead with amikacin 0.4 mg/0.1 ml [
19,
20,
21]. Some reports suggested an even greater susceptibility of Gram-negative bacteria to amikacin than ceftazidime [
22]. In the present study we found no resistance of Gram-negative bacteria to amikacin. Even though earlier endophthalmitis vitrectomy studies showed that intravitreal antibiotic injections should be the first line of treatment for cases of endophthalmitis following cataract surgery, newer studies show promising result and better visual outcomes as well as fewer total number of procedures when primary vitrectomy is performed [
19,
23,
24].
Evaluating the response to cephalosporins, cefuroxime (which is still often used intracameral as prevention therapy against bacterial infections after cataract surgery) displayed a moderate efficiency on Gram-positive bacteria and a weak therapeutic effect upon Gram-negative bacteria. Only 76.9% of the Gram-positive and 45.5% of the Gram-negative bacteria were sensitive to cefuroxime. Prophylactic intracameral cefuroxime has been used for over 20 years in the prevention of endophthalmitis after cataract surgery [
25]. When intraoperative anterior chamber irrigation of cefuroxime was compared to balanced salt solution, the rate of postoperative endophthalmitis after cataract surgery decreased by 7-fold [
26]. Multiple studies also showed a decreased incidence of endophthalmitis after introducing cefuroxime in their surgery protocol [
27,
28,
29,
30,
31]. Even though reports of routinely used intracameral cefuroxime for the prevention of postoperative endophthalmitis show good results, this protocol must be regularly reevaluated, and signs of antimicrobial resistance must be thoroughly reported [
12,
32]. Despite the fact that postoperative endophthalmitis incidence after usage of intracameral cefuroxime was reported to be only 0.033%, newer protocols which tested intracameral moxifloxacin or vancomycin showed even lower incidences (0.015% and 0.01%). This difference could be an indication of increased bacterial resistance to cefuroxime [
33]. A bacteriology profile and sensitivity to cefuroxime study conducted on a 20-year period showed an important shift to resistant organism. From the evaluated 20 years’ timeline, in the first period of the study endophthalmitis after cataract surgery was mainly caused by coagulase-negative Staphylococci, Staphylococcus aureus and Streptococci, while in the second period of the study, after introduction of intracameral cefuroxime, the incidence of enterococci and cefuroxime-resistant bacteria increased significantly [
30]. Other studies also reported endophthalmitis with cefuroxime-resistant bacteria after phacoemulsification [
34,
35,
36,
37]. Some clinical trials have suggested that topical antibiotics used in addition to intracameral cefuroxime lowers the chance of post-operative endophthalmitis when compared to cefuroxime injection alone [
38]. Other studies with low or moderate risk of bias reported no difference between the two protocols [
33]. Surgeons must take into consideration that intraoperative administration of cefuroxime can reduce the bacterial infection rate but also should not ignore the possibility of Gram-positive resistance development or the Gram-negative weak effect of the antibiotic [
39]. Ceftriaxone, another antibiotic evaluated from the cephalosporin class, showed moderate effect on Gram-positive bacteria (only 72.72% of the bacteria sensitive to the antibiotic) but excellent effect on Gram-negative bacteria (100% of the bacteria were sensitive to the antibiotic). In the treatment of endophthalmitis, ceftriaxone (at concentrations more than 50 mg/dose) cannot be administered as an intravitreal medication due to ocular toxicity but can by utilized as a systemic adjuvant therapy [
40,
41]. Systemic delivery of ceftriaxone will produce intravitreal antibiotic levels which inhibit Streptococci and Enterobacteriaceae but not Staphylococcus aureus [
42].
Our study showed that carbapenems are a highly efficient antibiotic class against both Gram-positive and Gram-negative bacterial (100% of the Gram-negative probes and over 90% of the Gram-positive probes were sensitive to meropenem and imipenem). For ocular infections, carbapenems are mainly used as an adjuvant systemic therapy in the treatment of endophthalmitis as it achieves efficient vitreous concentrations that are well above the necessary breakpoints of the Gram positive and negative bacteria [
43]. On the other hand, despite reports of proper intravitreal concentration of meropenem after systemic delivery, some studies show no additional benefit in visual outcome when compared to conventional systemic antibiotics in the treatment of postoperative endophthalmitis [
44]. Experimental models that evaluated topical meropenem in comparison to other antibiotic treatments showed promising result for the treatment of ocular infections with Pseudomonas [
45]. Studies also revealed that administration of topical meropenem as bacterial keratitis treatment has good corneal penetration as well as low toxicity. Besides Pseudomonas, the efficacy of meropenem was proved on Staphylococcus aureus, coagulase-negative staphylococci, streptococcus and Enterobacteriaceae infections [
46]. In vitro studies have shown higher susceptibility of both Gram-positive (including Methicilin-Resistent Coagulase -negative Staphylococcus) and Gram-negative bacteria to imipenem when compared to linezolid, tigecycline or fluoroquinolones [
47]. Also, attempts to treat post-traumatic Pseudomonas endophthalmitis through intravitreal injections of meropenem have shown, in experimental models promising result when compared to intravitreal ceftazidime [
48]. An important aspect to be noted is that meropenem, unlike ceftazidime, necessitates 3 intravitreal doses. Even though, meropenem is a potent antibiotic, very few studies analyze its therapeutic effect, adverse reactions on the retina and other ocular structures and the remanence time in the vitreous. The latter point appears to be one of the major problems that must be further studied, as meropenem has been reported in one study to have a half-life (t(1/2)) in the vitreous of only 2.6 h [
49]. To review, topical carbapenems emerge as a potent treatment for severe ocular surface infections, as its efficacy has been demonstrated to be great on both Gram- positive and -negative organisms, but few studies are available that assess any adverse effects, while treatment through intravitreal administration faces the problem of high vitreous washout dynamics.
In the present study, the therapeutic response to fluoroquinolones has been found to be moderate for Gram-positive bacteria (80.6%, 85.2%, 82.1% and 82.8% sensitive to moxifloxacin, levofloxacin, ofloxacin and ciprofloxacin) and excellent for Gram-negative organisms (100% sensitive for all four tested agents). Adequate efficiency on microorganisms of fluoroquinolones makes them appropriate as prevention treatment before surgery and decreases bacterial contamination. Nevertheless, prolonged use of antibiotics like fluoroquinolones induces antimicrobial resistance, which has become increasingly prevalent in situations of ocular infections [
50,
51,
52,
53]. Use of fluoroquinolones empirically as a broad-spectrum antibiotic for ocular infections as well as excessive administration in the perioperative period for procedures like intravitreal injections have increased its antimicrobial resistance. This is supported by result that reveal an increased risk of endophthalmitis after fluoroquinolones antibiotic prophylaxis due to selection of resistant conjunctival flora [
54,
55]. Resistance patterns depend not only on the type of antibiotic but also the regional prescription habits, with great variability between countries [
56]. In our study, increased resistance was observed for fluoroquinolones among Gram-positive bacteria, moxifloxacin being the least effective antimicrobial, even though it is a 4th generation fluoroquinolone. This trend was highlighted also by other recent studies [
52,
57]. Moreover, studies show that one-month prophylactic treatment of levofloxacin after cataract intervention yields fluroquinolone-resistant microorganisms [
58]. While reports on ciprofloxacin resistant Gram-positive bacteria have been also observed, other fluoroquinolones like besifloxacin have shown good therapeutic result in ocular infections with Staphylococcus aureus or Methicillin-Resistant Staphylococcus epidermidis [
59]. Other studies likewise observed no difference in final outcome between moxifloxacin and fortified vancomycin in the treatment of methicillin-resistant Staphylococcus aureus [
60]. For Gram-negative bacteria, fluroquinolones are extremely useful in the prevention of endophthalmitis before surgery. Fluoroquinolones were considered, in general, not suitable for intravitreal injections due to their toxicity. Still, few cases have been published showing good results after intravitreal injections of moxifloxacin for the treatment of acute post-operative endophthalmitis or for the prevention of it, following cataract surgery [
61,
62]. While some scientific papers reveal the tendency of moxifloxacin to exhibit increased antimicrobial resistance, others provide evidence that intracameral moxifloxacin could be a proper alternative for the prevention of endophthalmitis after surgery. As highlighted previously, some of the regularly used intracameral antibiotics (especially cefuroxime) raise concerns of bacterial resistance. Therefore, moxifloxacin is being investigated as a more adequate option. When compared to topical antibiotics given after surgery, intracameral moxifloxacin has shown a reduction of the endophthalmitis incidence rate by up to 4-fold. Nevertheless, when compared to intracameral cefuroxime, statistics indicated no significant benefit [
62,
63,
64,
65].
Aminoglycosides have long been used for the treatment of ocular infections both as a topical and as intravitreal medication. In our study, Gram-positive bacteria showed quite high resistance to aminoglycosides, mostly for tobramycin and kanamycin (only 72.9%, respectively 62.5% of the bacteria were sensitive to them). Netilmicin was the most effective antibiotic tested on Gram-positive organisms (94.7%). Regarding the Gram-negative bacteria, tobramycin was the least effective antimicrobial, while 100% were sensitive to amikacin, kanamycin, and gentamicin. Overall, this antibiotic class showed moderate potency on Gram-positive microorganisms and a better efficacy on Gram-negative. Retrospective studies have also confirmed a trend of antimicrobial resistance of aminoglycosides on Gram-positive, especially Staphylococcus aureus and Coagulase-Negative Staphylococcus, which are among the most frequent pathogens involved in ocular infections [
66,
67]. Not all studies reported a progressive increase of tobramycin bacterial resistance. This aspect is highly dependent on the prescription pattern of the region. In US, the Antibiotic Resistance Monitoring in Ocular Microorganisms (ARMOR) surveillance study reported a small decrease of the Staphylococcus aureus resistance rates to tobramycin at least for the timeline 2009-2016 [
68]. While other antibiotic classes penetrate the aqueous humor after topical administration, netilmicin and tobramycin do not reach detectable concentrations. Thus, they are not adequate for intraocular infections when used as topical treatment [
69]. For the prophylaxis of endophthalmitis in patients that underwent cataract surgery, topical aminoglycosides were ineffective when compared to gatifloxacin [
70]. On the other hand, gentamicin attained good out-turn in reducing the incidence of endophthalmitis when injected into the surgical perfusion solution. Still, some result indicate that gentamicin may ease the development of resistant strains of Enterococcus [
71]. For the prophylaxis of endophthalmitis after intravitreal injections with anti-VEGF, aminoglycosides are not indicated, especially when they are associated with corticosteroids, as the risk of infection is actually higher for the later, when compared to no antibiotic prophylaxis [
72,
73]. Aminoglycosides can be used as intravitreal treatment being both effective and with a much lower toxicity level compared to fluoroquinolones. Nevertheless, caution still must be expressed regarding the retinal toxicity.
We observed a good efficacy of chloramphenicol for both Gram-positive and negative bacteria. We cannot assess with high accuracy the efficiency on Gram-negative bacteria due to the small sample of culture tests. Nevertheless, chloramphenicol remains a viable option as prevention treatment before and after ocular surgery and is still an active antibiotic against ocular infections [
74,
75]. It has also shown limited bacterial resistance when compared to other broadly used antibiotics like fluoroquinolones [
76,
77,
78]. A 30-year study, that focused on antimicrobial resistance trends showed that overall, chloramphenicol is one of the most effective antibiotics against bacterial ocular infections [
79]. Even more severe eye infections induced by Methicillin-Resistant Staphylococcus aureus, showed a general good response to chloramphenicol and an uncommon bacterial resistance [
77,
80,
81,
82]. For chloramphenicol-resistant microorganisms, newer derivates like chloramphenicol-borate have emerged as a potential new antibiotic treatment [
83]. In recent years the use of chloramphenicol has been reestablished after a period of reduced usage due to reports of aplastic anemia adverse reactions [
84,
85].
Tetracyclines have been used most commonly in ophthalmological practice as topical or oral antibiotics for the treatment of Chlamydia trachomatis, which is a Gram-negative bacterium that determines one of the leading infectious causes of blindness in the world [
86]. No significant difference was detected for the treatment outcome of active trachoma at 3 and 12 months when oral and topical treatments were compared. On the other hand, in the present study, we reported 14.3% resistance of Gram-negative microorganisms to tetracycline and zero to doxycycline. Increased resistance was observed for Gram-positive microorganisms (47.1%, respectively 47.4% of the reported samples were sensible to tetracycline, respectively to doxycycline). This trend of increased bacterial resistance is in accordance with other recent studies [
87,
88,
89,
90]. Nevertheless, reports have shown that for certain bacterial species like Coagulase-negative Staphylococci the resistance rates to tetracycline have dropped [
91]. Resistance report of Chlamydia trachomatis to tetracycline as scarce, even though they seem to be increasing [
86,
92]. Beside the antibacterial effect, tetracyclines exhibit anti-inflammatory properties that renders them appropriate as an adjunctive therapy for posterior blepharitis or rosacea associated ocular manifestations [
93,
94,
95,
96,
97,
98].
Macrolides (azithromycin, erythromycin and clarithromycin being the most prescribed) have been used in ophthalmological practice for the treatment of Chlamydia trachomatis and Gonococcus ocular infections. They have been administrated in the form of oral tablets, ophthalmic ointment, topical gel, or eye drops solutions [
99,
100,
101,
102]. In the present study we found no resistance of the Gram-negative bacteria for azithromycin and clarithromycin. Even though macrolides have been characterized as having a broad antimicrobial spectrum, we found reduced sensitivity of Gram-positive bacteria to this antibiotic class, clarithromycin being the most effective, while azithromycin being the least efficient. Penicillins are not usually used for ocular infections due to the increased bacterial resistance.
The limitations of the study are the inhomogeneous testing settings (not all laboratories used the same antibiotic set), the relative smaller number of Gram-negative bacteria tested. The lower number of Gram-negative samples is due to the fact that Gram-positive contamination of the ocular surface and lid margin is more frequent.
The novelties and contributions of the present study are un updated picture of the dynamics of antibiotic treatments in bacterial contamination of the ocular surface and an increased awareness of the antimicrobial resistance that cefuroxime developed due to long-term usage as an intracameral prevention treatment for endophthalmitis. Also, we report an increased bacterial resistance to moxifloxacin, another frequently used broad-spectrum antibiotic. Knowledge of the regional rates of bacterial resistance to antibiotic is extremely important for proper management of ophthalmologic infections. Also, the current study illustrates the real life setting and offers guidance for the prevention and treatment of ocular infections.