In Vitro Antibiotic Susceptibilities of Francisella tularensis Determined by Broth Microdilution following CLSI Methods

Abstract

In vitro susceptibilities for 47 antibiotics were determined in 30 genetic diverse strains of Francisella tularensis by the broth microdilution method following Clinical and Laboratory Standards Institute (CLSI) methods. The F. tularensis strains demonstrated susceptibility to aminoglycosides, fluoroquinolones, and tetracyclines. There was a distinct difference in macrolide susceptibilities between A and B type strains, as has been noted previously. The establishment and comparison of antibiotic susceptibilities of a diverse but specific set of F. tularensis strains by standardized methods and the establishment of population ranges and MIC 50/90 values provide reference information for assessing new antibiotic agents and a baseline to monitor any future emergence of resistance, whether natural or intentional.KEYWORDS Francisella tularensis, antibiotics, susceptibility testing F rancisella tularensis is the causative agent of tularemia, a zoonotic infection in humans, usually transmitted as a result of contact with infected animals or insect bites (1). It also has potential misuse in biowarfare and bioterrorism (2). While the disease is not usually fatal, underlying medical conditions can lead to possibly fatal complications. Antibiotic therapy usually resolves the infection (1). The species is also divided into several biovars, based on metabolic differences, virulence, and geographic location (3). There is scattered information on MICs under a variety of nonstandardized testing conditions, with mostly type B (F. tularensis subsp. holarctica) strains (4-15). A more recent report utilizing standardized testing methods with a variety of North American type A strains (16) has provided some additional data; however, this study looked at only eight antibiotics representing five classes. With a lack of comparative data on type strains and with a variety of antibiotic classes, we report here specific antibiotic susceptibility results according to Clinical and Laboratory Standards Institute (CLSI) microdilution broth methodology for 30 strains of F. tularensis (17). This information will be highly useful as baseline data in the event of wartime or terrorist release, as well as for naturally acquired and laboratory-acquired infections.The F. tularensis strains used in this study are shown in Table 1, and they were obtained from the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID) collection and selected to represent the established biovars and geographic diversity (18). Most antibiotics were obtained from U.S. Pharmacopoeia (Rockville, MD), with the following exceptions: ceftriaxone and fusidic acid were from Sigma Chemical Co. (St. Louis, MO), cethromycin was from Advanced Life Sciences; telithromycin was from Sanofi-Aventis, garenoxacin was from Schering-Plough, gemifloxacin was from Oscient, ertapenem was from Merck, faropenem was from Replidyne, and tigecycline was from Wyeth. Most stock solutions (5 mg/ml) were prepared for each drug in the appropriate solvents, based on the current CLS...

A Survey of Antimicrobial Resistance Determinants in Category A Select Agents, Exempt Strains, and Near-Neighbor Species

“…QRDRs in gyrA and parC were assessed by HRMA in seven F. tularensis and three F. philomiragia strains using PCR primers designed to bracket mutations observed to result in ciprofloxacin resistance [9,34,35]. In the gyrA assay, all F. tularensis strains clustered together (Table 5, Figure 4); these results are consistent with the phenotypic susceptibility of all strains for which ASTs have been reported [36]. Formation of a separate cluster by all three F. philomiragia, with T m s nearly a degree higher, was not surprising.…”

Antimicrobial Susceptibility Test for the Determination of Resistant and Susceptible S. aureus and Enterococcus spp. Using a Multi-Channel Surface Plasmon Resonance Device

“…Various standardized phenotypic antimicrobial susceptibility testing methods such as agar diffusion [11], broth microdilution [12], gradient strip tests [13], and disk diffusion [14] have traditionally been used by researchers to determine whether the bacteria are resistant or susceptible. Resistance to specific antibiotics can also be determined by PCR, which investigates the presence or absence of genes responsible for the resistance [15].…”

Genetic Determinants of Antibiotic Resistance in Francisella

“…Unfortunately, the strains comparison across studies is difficult as the nomenclature of Francisella has changed considerably over the years, and many strains are annotated only as F. tularensis . Other factors precluding direct comparison include different testing methods of AMR, such as non-CLSI standard minimum inhibitory concentration (MIC) testing ( Heine et al, 2017 ) and E-strip testing and the use of multiple media with various supplementations.…”