Antibiotic resistance is one of the most pressing challenges to global health today, resulting in longer hospital stays, higher medical costs and increased mortality, according to the WHO.1 In the United States alone, at least 2.8 million people contract an antibiotic-resistant infection, and more than 35,000 people die.2 As microbes quickly develop new resistance mechanisms, antibiotic treatments lose their effectiveness to fight infections such as pneumonia, tuberculosis, blood poisoning and foodborne diseases.1 The main driver of this problem, the WHO points out, is the way antibiotics are prescribed and used—too frequently on both counts.1

The long-running Antibiotic Resistance Monitoring in Ocular micRoorganisms (ARMOR) surveillance study informs clinicians of antimicrobial susceptibility patterns in ocular bacterial pathogens.3 Among ocular infections, gram-positive bacteria such as staphylococci are the most prevalent culprits.

“Resistance in these bacteria may lead to treatment failure,” said Penny Asbell, MD, of the University of Tennessee Health Science Center and director of the Hamilton Eye Institute, in her presentation on the latest results, presented yesterday during a virtual ARVO conference. “If we understand antibiotic resistance, we can pick more wisely which antibiotic to begin with and which one to switch to if needed.”

The ARMOR investigators analyzed in vitro resistance among staphylococcal isolates collected between 2009 and 2020.3 The 12-year results presented at the ARVO 2021 virtual meeting indicated that staphylococci generally had minimal or no change in in vitro antibiotic resistance.

In the ARMOR trial, researchers performed susceptibility testing on Staphylococcus aureus and coagulase-negative staphylococci (CoNS). The minimum inhibitory concentration was defined as susceptible/resistant to 16 different antibiotics based on Clinical and Laboratory Standards Institute methods and breakpoints.

They found a significant decreasing trend in methicillin resistance (MR) among S. aureus but no change in MR among CoNS. Other significant decreasing trends were observed in resistance to azithromycin, ciprofloxacin and tobramycin, and to ciprofloxacin in CoNS. Trends in increasing resistance were reported in tetracycline among S. aureus and in trimethoprim among CoNS.

Multidrug resistance (three or more antibiotic classes) among MR strains remained prevalent in 2020, the researchers reported. No isolates were vancomycin-resistant, and besifloxacin retained “consistently low” minimum inhibitory concentrations during the study period.

 “MR is often a marker for multidrug resistance in these organisms,” Dr. Asbell said. “This surveillance study gives us the big picture. I think it can help us determine clinically what antibiotic we might start with, but clearly, clinical presentation is going to be equally important in deciding how we treat patients. The ARMOR study gives us a key piece of information for us to use in our armamentarium as we try to treat patients wisely.”

The investigators concluded that staphylococci demonstrated only minimal changes in antibiotic resistance; however, they cautioned about the small decrease in methicillin resistance in S. aureus, noting, “The high level of methicillin resistance in staphylococci warrants attention when selecting empiric antibiotic therapy, particularly with respect to multidrug resistance in these organisms.”

Cumulative in Vitro MIC90s* for Staphylococci Collected in ARMOR (2009-2020)3

Antibiotic S. aureus (n=2599) MRSA (n=900) CoNS (n=2143)MR CoNS (n=1041) 
Moxifloxacin16 1632 
Levofloxacin16128128 256 
 Ofloxacin>83216 256 
*MIC90: minimum concentration that inhibits 90% of isolates (µm/mL)

1. Fact Sheet: Antibiotic Resistance. Accessed May 3, 2021.

2. Antibiotic/Antimicrobial Resistance (AR/AMR). Accessed May 3, 2021.

3. Asbell PA, Sanfilippo CM, DeCory HH. Analysis of longitudinal antibiotic susceptibility trends in Staphylococci: results from 12 years of the ARMOR Study. ARVO Meeting 2021.