Microsoft word - antimicrobial susceptibility - viridans streptococci.doc

ANTIMICROBIAL SUSCEPTIBILITY – PENICILLIN RESISTANT VIRIDANS STREPTOCOCCI
This specimen was a simulated blood culture to be processed by aerobic organism culture for identification and by routine antimicrobial susceptibility testing. The organism (viridans group streptococcus, S. mitis / oralis) was highly resistant to β-lactam antimicrobial agents (penicillins and cephalosporins), commonly used for therapy of infections caused by these species. This organism challenge was sent as an educational exercise and grading was not applied. The following participant responses (≥5) were considered to be satisfactory identifications, if this specimen had been graded (no./%): alpha strep, not S. pneumoniae (254/40.1%); S. mitis (188/29.7%); Streptococcus, alpha hemolytic (75/11.8%); aerobic growth, referred (45/7.1%); Streptococcus spp. (26/4.1%); and Streptococcus spp. (viridans group) (16/2.5%). The most common erroneous responses were: Gemella morbillorum (1.1%), S. pneumoniae (0.8%), and Enterococcus spp. (0.5%). Alpha-hemolytic streptococcal organisms have long been the subject of taxonomic study and continued nomenclature change. This organism was morphologically consistent with a Streptococcus spp. by showing negative catalase and bile-solubility tests as well as Gram-positive cocci in chains. The cells were ≤2 µm and on media containing sheep blood exhibited greening or alpha-hemolysis. The commonly used Vitek 2 identification system produced a profile number with a 99.0% probability of S. mitis / oralis. Currently, the so-called “S. mitis group” includes S. mitis, S. sanguinis, S. parasanguinis, S. gordonii, S. cristatus, S. oralis, S. infantis, and S. peroris (1-3). These species are generally difficult to differentiate (1) and specific attempts at precise identification should be limited to those isolates from serious infections including deep abscesses, osteomyelitis, endocarditis, and bacteremias (sepsis, as in this specimen) in immunocompromised or neutropenic patients. Occasional transient bacteremias with viridans group species are noted that resolve without serious sequelae. Fatal outcomes, however, have occurred from sustained bloodstream infections with S. mitis in neutropenic cancer patients, neonates, and as an associated complication of adult respiratory distress syndrome (1, 4). Increasing resistance to penicillins, cephalosporins, macrolides, fluoroquinolones, and other antimicrobial families requires accurate susceptibility testing (5-9) for this organism group. Resistances to β-lactams can be striking due to markedly altered penicillin-binding-proteins (PBPs) and similarly, elevated macrolide MIC results follow altered 23S rRNA targets (erm-mediated methylases) or efflux pumps (mefA and others) (1). More recently, gene mutations in the quinolone resistance determining region (QRDR) produce modest to highly elevated MIC values for the fluoroquinolones (ciprofloxacin, gatifloxacin, gemifloxacin, levofloxacin, moxifloxacin) that can also cause treatment failures in streptococcal infections (10). These later resistances are more rare, usually <5%. American Proficiency Institute – 2008 1st Test Event ANTIMICROBIAL SUSCEPTIBILITY – PENICILLIN RESISTANT VIRIDANS STREPTOCOCCI (cont.)
The susceptibility testing of streptococci has been standardized and described in detail in CLSI documents (M2-A9, M7-A7, M100-S18) (6-8). MIC tests require the use of Mueller-Hinton broth (CAMHB) with lysed horse blood (2.5-5% V/V) and CAMHB supplemented to 50 mg/L calcium, if testing daptomycin. For the disk diffusion (DD) method, Mueller-Hinton agar with 5% sheep blood is required. When testing viridans group streptococci (Tables 2H-2 of M100-S18), the incubation environment is 35 ± 2ºC in 5% CO2 and endpoints determined after 20-24 hours. Recently, Table 24H was divided into two components with Table 2H-2 listing the breakpoints for “Streptococcus spp. Viridans group”. For the reference MIC method, Table 2H-2 contains breakpoint criteria for 22 antimicrobials. That document states that “susceptibility to penicillin” can predict susceptibility to several other listed β-lactam agents (21 total). In contrast, penicillin and ampicillin DD tests are not optimal and “viridans streptococci isolated from normally sterile body sites should be tested for penicillin susceptibility using an MIC method”. Eighteen antimicrobials have zone diameter breakpoint criteria (8). Close examination of the M2 and M7 Tables 2H-2 shows that only MIC criteria are published for ertapenem, meropenem and daptomycin, while only DD breakpoints are listed for grepafloxacin, a fluoroquinolone very uncommonly The challenge organism (ES-01, 2008) was only susceptible to chloramphenicol, daptomycin, levofloxacin, linezolid and vancomycin (Table 1) among those compounds that have CLSI interpretive guidelines. All other agents were not therapeutic options, including penicillin (MIC, >32 µg/ml), macrolides (MIC, >2 µg/ml), and ceftriaxone (MIC, >32 µg/ml). Among the agents where no interpretive breakpoints exist, moxifloxacin (MIC, ≤0.5 µg/ml), telithromycin (MIC, ≤0.5 µg/ml) and tigecycline (MIC, 0.06 µg/ml) all exhibited high potencies (Table 1). Table 2 summarizes the performance of DD and MIC tests by category of susceptibility compared to reference CLSI MIC results (Table 1). Both methods performed well (93.1-100.0% and 85.0-100.0% for DD and MIC, respectively). Of greatest concern were the interpretive categories reported by DD test users for ampicillin, penicillin, gentamicin, and trimethoprim/sulfamethoxazole (TMP/SMX) where no breakpoints have been published by the CLSI (8). Similarly, participants using MIC reported gentamicin and TMP/SMX results, again without CLSI guidance. If graded, such practices would be penalized regardless of appropriate categorization. CLSI Table 1 recommends testing the following agents against clinically relevant bloodstream isolates of viridans group streptococci: penicillin or ampicillin, cefepime or cefotaxime or ceftriaxone, vancomycin, chloramphenicol, clindamycin, erythromycin and linezolid. Please note that erythromycin testing can predict susceptibility and resistance to azithromycin, clarithromycin and dirithromycin. Also, inducible clindamycin resistance should be assessed by D-test for erythromycin-resistant clindamycin-susceptible strains, if clindamycin were considered for treatment. American Proficiency Institute – 2008 1st Test Event ANTIMICROBIAL SUSCEPTIBILITY – PENICILLIN RESISTANT VIRIDANS STREPTOCOCCI (cont.)
Table 1. Listing of expected susceptibility testing category results for a viridans group streptococcus
(S. mitis / oralis group [S. peroris]) in ES-01 (2008).
Antimicrobials by susceptibility category (reference MIC in µg/ml): Clindamycin (>2) Piperacillin/tazobactam a. Antimicrobials without CLSI interpretive criteria (M100-S18, 2008).
Table 2. Participant performance for selected agents tested by agar disk diffusion (DD) or quantitative
MIC methods for ES-01 (2008), a viridans group streptococcus (only agents with ≥10 responses for
either test method).
a. Responses for antimicrobials without CLSI interpretive criteria (M100-S18, 2008). American Proficiency Institute – 2008 1st Test Event ANTIMICROBIAL SUSCEPTIBILITY – PENICILLIN RESISTANT VIRIDANS STREPTOCOCCI (cont.)
References:
Spellerberg B, Brandt C. Streptococcus. In: Murray PR, Baron EJ, Jorgensen JH, Landry ML, Pfaller MA, editors. Manual of Clinical Microbiology, 9th ed. Washington, D.C.: ASM Press; 2007. p. 412- Kawamura Y, Hou XG, Todome Y, Sultana F, Hirose K, Shu SE, et al. Streptococcus peroris sp. nov. and Streptococcus infantis sp. nov., new members of the Streptococcus mitis group, isolated from human clinical specimens. Int J Syst Bacteriol. 1998 Jul;48 (Pt 3):921-7. Kawamura Y, Hou XG, Sultana F, Miura H, Ezaki T. Determination of 16S rRNA sequences of Streptococcus mitis and Streptococcus gordonii and phylogenetic relationships among members of the genus Streptococcus. Int J Syst Bacteriol. 1995 Apr;45(2):406-8. Bochud PY, Calandra T, Francioli P. Bacteremia due to viridans streptococci in neutropenic patients: A review. Am J Med. 1994 Sep;97(3):256-64. Doern GV, Ferraro MJ, Brueggemann AB, Ruoff KL. Emergence of high rates of antimicrobial resistance among viridans group streptococci in the United States. Antimicrob Agents Chemother. 1996 Clinical and Laboratory Standards Institute. M2-A9. Performance standards for antimicrobial disk susceptibility tests; approved standard - ninth edition. Wayne, PA: CLSI, 2006. Clinical and Laboratory Standards Institute. M7-A7, Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically; approved standard - seventh edition. Wayne, PA: Clinical and Laboratory Standards Institute. M100-S18, Performance standards for antimicrobial susceptibility testing, 18th informational supplement. Wayne, PA: CLSI, 2008. Moet GJ, Dowzicky MJ, Jones RN. Tigecycline (GAR-936) activity against Streptococcus gallolyticus (bovis) and viridans group streptococci. Diagn Microbiol Infect Dis. 2007 Mar;57(3):333-6. Davidson R, Cavalcanti R, Brunton JL, Bast DJ, de Azavedo JC, Kibsey P, et al. Resistance to levofloxacin and failure of treatment of pneumococcal pneumonia. N Engl J Med. 2002 Mar American Proficiency Institute – 2008 1st Test Event

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