Minimal inhibitory concentration (MIC) is the lowest concentration of antibiotics that inhibits the visible growth of bacteria. It has been reported that sub-MIC of antibiotics may result in morphological alterations, along with the biochemical and physiological changes in bacteria. The purpose of this study was to examine morphological changes of Aggregatibacter actinomycetemcomitans, after the treatment with sub-MIC metronidazole and penicillin. The bacterial morphology was observed with scanning electron microscope, after incubating with sub-MIC antibiotics. The length of A. actinomycetemcomitans was increased after the incubation with sub-MIC metronidazole and penicillin. Sub-MIC metronidazole and penicillin inhibited bacterial division and induced long filaments. Our study showed that metronidazole and penicillin can induce the morphological changes in A. actinomycetemcomitans.

The objective of this study was to develop a simultaneous method of 8 penicillin antibiotics including amoxicillin, ampicillin, cloxacillin, dicloxacillin, nafcillin, oxacillin, penicillin G and penicillin V in meat using LC-MS/MS. The procedure involves solid phase extraction with HLB cartridge and subsequent analysis by LC-MS/ MS. To optimize MS analytical condition of 8 compounds, each parameter was established by multiple reaction monitoring in positive ion mode. The chromatographic separation was achieved on a C18 column with a mobile phase of 0.05% formic acid and 0.05% formic acid in acetonitrile at a flow rate of 0.2 mL/min for 20 min with a gradient elution. The developed method was validated for specificity, linearity, accuracy and precision in beef, pork and chicken. The recoveries were 71.0~106%, and relative standard deviations (RSD) were 4.0~11.2%. The limit of detection (LOD) and the limit of quantification (LOQ) were 0.003~0.008 mg/kg and 0.01~0.03 mg/kg, respectively, that are below maximum residue limit (MRL) of the penicillins. This study also performed survey of residual penicillin antibiotics for 193 samples of beef, pork and chicken collected from 9 cities in Korea. Penicillins were not found in all the samples except a sample of pork which contained cloxacillin (concentration of 0.08 mg/kg) below the MRL (0.3 mg/kg).

A rapid, simple and reliable LC-MS/MS method, which can be used on a routine basis, was developed for the simultaneous detection of 8 penicillin antibiotics (amoxicillin, ampicillin, penicillin-G, penicillin-V, oxacillin, cloxacillin, nafcillin and dicloxacillin) in swine muscle and kidney. The antibiotics were extracted from samples with water and methanol. The extract was centrifuged, filtered and analyzed by liquid chromatography coupled with a tandem mass spectrometer (LC-MS/MS), using a C18 reversed phase column with water/acetonitrile gradient containing 0.05 % formic acid. Mass spectral acquisition was achieved in an electrospray positive ion mode by applying multiple reaction monitoring (MRM) of 2 fragment ion transitions to provide a high degree of sensitivity and specificity. The ion rations were consistent and could be used for confirmation of identity of the penicillin antibiotics. Recoveries of eight penicillins at three fortification levels (10, 20 and 40㎍/㎏) ranged from 79.8 to 102.4% and 72.8 to 103.4% in swine muscle and kidney, respectively. The coefficient of variation was than 9% in all samples. The estimated limits of quantification (LOQs) ranged from 1.0 to 3.2㎍/㎏ in swine muscle and kidney, respectively. The LOQs of this method are below the MRLs of penicillin antibiotics in animal tissues established in Korea and other countries.