Antibiotic susceptibility of Fusobacterium necrophorum isolated from liver abscesses

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Summary
Antibiotic susceptibility patterns of the primary causative agent of liver abscesses thirty-seven isolates of Fusobacterium in feedlot cattle.Two distinct biotypes or necrophorum (21 biotype A and 16 biotype subspecies, biotype A (subsp.necrophorum) B) from liver abscesses of feedlot cattle were and biotype B (subsp.funduliforme), have determined.These isolates were generally been recognized.Biotype A is encountered susceptible to penicillins, tetracyclines most frequently in liver abscesses.Because (chlortetracycline and oxytetracycline), of the importance of F. necrophorum as an lincosamides (clindamycin and lincomycin), animal pathogen, its antibiotic susceptibility, and macrolides (tylosin and erythromycin) particularly to clinically relevant antibiotics, but resistant to aminoglycosides (kanamycin, has been reported.However, studies on neomycin, gentamycin and streptomycin), susceptibility to antibiotics used as feed addiionophores (except narasin), and peptides tives have been limited.Also, the difference (avoparcin, polymixin, and thiopeptin).Dif-in susceptibility patterns between the two ferences in antibiotic sensitivity patterns biotypes have not been reported.Our objecwere observed between the two biotypes tives were to determine the susceptibility of only for clindamycin and lincomycin.The F. necrophorum of liver abscess origin to minimum inhibitory concentrations (MIC) of antibiotics, including FDA-approved and FDA-approved antibiotics for liver abscess certain experimental feed additives, and to control did not parallel their efficacy in pre-determine whether continuous antibiotic venting clinical liver abscesses in feedlot cat-feeding during the finishing period would tle.Continuous tylosin feeding did not ap-influence susceptibility of F. necrophorum to pear to induce antibiotic resistance in F.

Experimental Procedures
Kansas and eastern Missouri were collected at a slaughter house.Lot numbers and cattle resistance of F. necrophorum to ionophore origins were recorded to enable obtaining antibiotics explains the lack of influence of information on antibiotic feeding.Fuso-monensin or lasalocid on the incidence of bacterium necrophorum was isolated from liver abscesses.the abscesses.Thirty-seven isolates ( 21 1).necrophorum isolates to antibiotics was determined by inoculating overnight cultures Only 31 of the 37 isolates were from (16 to 18 hours) into anaerobic media with cattle with known antibiotic feeding status 100 µg/ml or units/ml (for bacitracin, during the finishing period.Twenty-three penicillin-G, polymyxin-B) of antibiotics or isolates were from tylosin-fed (10 g/ton of no antibiotic.Culture absorbance was mea-feed) cattle and eight were from cattle that sured three times to determine growth.For did not receive tylosin.Only four isolates antibiotics that were inhibitory at 100 µg/ml were from chlortetracycline-fed (75 or units/ml, the minimum inhibitory concen-mg/head/day) cattle.The mean MICs for tration (MIC) was determined by broth tylosin were similar whether or not the cattle microdilution.The MIC was the lowest had been fed tylosin (Table 2).Similarly, concentration of the antibiotic that inhibited continuous feeding of chlortetracycline had growth.Differences in MIC between the two no effect on the MIC of either biotypes, between tylosin-and nontylosin-fed chlortetracycline or oxytetracycline (Table 2).cattle, and between chlortetracycline-and Apparently, continuous feeding of tylosin or nonchlortetracycline-fed cattle were com-chlortetracycline did not induce antibiotic pared by a statistical t test.

Results and Discussion
Fusobacterium necrophorum isolates chlortetracycline, oxytetracycline, tylosin, and from liver abscesses were resistant (100 µg virginiamycin) are approved for use in the or units/ml) to avoparcin, gentamycin, prevention of liver abscesses in feedlot cattle.kanamycin, lasalocid, monensin, nalidixic acid, neomycin, salinomycin, streptomycin, tetronasin, thiopeptin, and vancomycin.The and lincomycin, MICs were lower (P < 0.05) According to the U.S. Feed Additive Compendium, five antibiotics (bacitracin, Based on MIC, chlortetracycline and oxytetracycline were most effective and bacitracin was the least effective.However tylosin is most effective in preventing clinical liver clinical efficacy.The mode of action of these abscesses.Except for bacitracin, MIC does antibiotics in preventing liver abscesses not appear related to is possibly inhibition or reduction of the population of F. necrophorum in ruminal contents and (or) in the liver.Tylosin was fed at 10 g/ton of feed throughout the finishing period.
a Chlortetracycline was fed at 75 mg/head/day throughout the finishing period. b

2
Department of Diagnostic Medicine/Pathology and Microbiology.

Table 1 .
The MICs of all antibiotic comtetronasin, thiopeptin, tylosin, vancomycin, pounds except for clindamycin and and virginiamycin.Antibiotic type is shown lincomycin did not differ between the two in Table1.biotypes of F necrophorum.For clindamycin Susceptibility or resistance of F. for biotype A than biotype B (Table

Table 1 . Mean Minimum Inhibitory Concentrations (MICs) of Antibiotics for Fusobacterium necrophorum Isolates from Liver Abscesses a
a Concentrations in µg/ml except for bacitracin, penicillin-G, and polymyxin-B, which are in units/ml.bDifferent from biotype A.c