Antimicrobial susceptibility of bifidobacteria A.M. Yazid 1 , A.M. Ali 2 , M. Shuhaimi 1 , V. Kalaivaani 1 , M.Y. Rokiah 3 and A. Reezal 4 1 Departments of Food Technology, 2 Biotechnology, 3 Nutrition and Community Health and 4 Food Science, Universiti Putra Malaysia, Serdang, Selangor, Malaysia 2238/99: received 28 September 1999, revised 11 February 2000 and accepted 4 April 2000 A.M. YAZID, A.M. ALI, M. SHUHAIMI, V. KALAIVAANI, M.Y. ROKIAH AND A. REEZAL. 2000. Eighteen Bi®dobacterium strains were tested for their susceptibility to a range of antimicrobial agents. All the strains tested, including the reference culture Lactobacillus acidophilus CH2, were susceptible to several groups of antimicrobial agents, they were cephalosporin (cefamandole, cefazolin, cefaperazone, cefoxitin), polypeptide (bacitracin), macrolide (erythromycin), penicillin (amoxicillin), phenicol (chloramphenicol) and b-lactam (imipenem). Fourteen strains were resistant to more than 10 antibiotics. The reference culture was resistant to only three antibiotics. The results showed that bi®dobacteria are resistant to a wide range of antimicrobial agents. INTRODUCTION Bi®dobacteria were ®rst isolated from the faeces of breast- fed infants by Tissier (1900), who used the name Bacillus bi®dus communis. Bi®dobacteria are Gram-positive, non- gas-producing, anaerobes with bi®d morphology. Today, this genus, which belongs to the Actinomycetaceae group, includes 30 species (Ishibashi et al. 1997). Bi®dobacteria are known as probiotic organisms because of the potential bene®cial roles of bi®dobacteria in the intestinal tract of humans (Hughes and Hoover 1991). These bacteria were shown to play a signi®cant role in controlling acidity of the large intestinal tract and capable of hydrolysing indigestible complex carbohydrate such as lactulose, into acetic and lac- tic acids. These acids are responsible for maintaining the intestinal microbial balance by inhibiting the growth of potential pathogens (Rasic 1983). Antimicrobial susceptibility of intestinal micro-organ- isms is an important criterion for selecting an organism as probiotic. The administration of antimicrobial substances can alter the intestinal microbial balance and suppress cer- tain bene®cial bacterial groups, including bi®dobacteria. The altered microbial balance may result in intestinal dis- orders (Kobayashi et al. 1973). The susceptibility of bi®do- bacteria to various antimicrobial agents are of interest in understanding the alteration of normal intestinal micro¯ora when antimicrobial agents were taken. The present study was conducted to determine the susceptibility of various strains of bi®dobacteria to several groups of common anti- microbial agents. MATERIALS AND METHODS Micro-organisms Eighteen strains representing 10 species of bi®dobacteria were obtained from American Type Culture Collection (Rockville, MD, USA). Lact. acidophilus CH2 were isolated from a commercial starter culture (Chr. Hansen, Hùrsholm, Denmark) and used as a control. Media and antimicrobial agents Test organisms were maintained and propagated in trypti- case phytone yeast extract (TPY) medium as recommended by Scardovi (1986). The following groups of antimicrobial agents were used: cephalosporin, sulphonamide, tetracy- cline, polypeptide, macrolide, nitrofurantoin, penicillin, phenicol, aminoglycoside, b-lactam and others. Antimicrobial susceptibility test A suspension of 10 8 cells ml 1 was used as inoculum (Lehto and Salminen 1997) and sensitivity testing was done by Stokes disc diffusion technique (Stokes and Ridgway 1987). Standard discs of 36 antimicrobial agents (Beckton Dickinson, Mountain View, CA, USA) were seeded in the plates and incubated anaerobically at 37  C for 48 h. Zone of inhibition was measured and the mean of two readings was converted to the nearest millimetre. Correspondence to: A.M. Yazid, Department of Food Technology, Faculty of Food Science and Biotechnology, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia (e-mail: myazid@fsb.upm.edu.my). Letters in Applied Microbiology 2000, 31, 5762 = 2000 The Society for Applied Microbiology