Abstract: Objective To investigate the cross-resistance among the different fluroquinolones (FQs) in oflo-xacin-resistant Mycobacterium tuberculosis(M. tuberculosis) isolates.  Methods One hundred and thirty ofloxacin-resistant M. tuberculosis isolates were selected from national tuberculosis drug-resistant survey conducted in 2007. Broth dilution method was used to detect the minimal inhibitory concentration (MIC) of M. tuberculosis against five FQs, including ofloxacin (Ofx), levofloxacin (Lfx), moxifloxacin (Mfx), gatifloxacin (Gfx) and sparfloxacin (Sfx). Chi square test was performed to compared the drug resistant rates of different FQs. The difference was considered as significant when P<0.05.  Results Of 130 Ofx-resistant M. tuberculosis isolates, there were 110 (84.6%，110/130), 108 (83.1%，108/130), 46 (35.4%，46/130) and 66 (50.8%，66/130) isolates resistant to Lfx, Mfx, Gfx and Sfx, respectively. Statistical analysis revealed that the Gfx-resistant rate of M. tuberculosis isolates was significantly lower than those of Lfx（χ²=65.641, P<0.01）, Mfx（χ²=61.225, P<0.01） and Sfx（χ²=6.274, P=0.02）. Among the Lfx-resistant isolates, 90.9%（100/110）, 41.8%（46/110） and 59.1%（65/110） were resistant to Mfx, Gfx and Sfx, respectively. In addition, there were 97.8% (45/46) and 97.0% (64/66) of Gfx- and Sfx-resistant isolates resistant to Mfx, respectively. For Gfx- and Sfx-resistant isolates, only 41.7%（45/108） and 59.3%（64/108） showed resistance to Mfx, respectively.  Conclusion  Our findings demonstrate that the Gfx- and Sfx-resistant rates were significantly lower than those of the others. In addition, the cross-resistance is observed between Lfx and Mfx, while there is partially unidirectional cross-resistance between Mfx and Gfx/Sfx.

Key words: Mycobacterium tuberculosis, Fluoroquinolones, Ofloxacin, Drug resistance, bacterial