Metronidazole suppress protozoans such as Entamoeba histolytica, Giardia lamblia and Trichomonas vaginalis. Further studies show that Metronidazole has been used to repress the growth of gram-negative anaerobes belonging to the Bacteroides and Fusobacterium, and gram-positive anaerobes such as peptostreptococcus and Clostridia. The advantages of this antibiotic is that it affects a high percentage of gram-negative bacteria and has greater tissue penetration. Furthermore, the gene hefA codes for the TolC efflux pump in Helicobacter pylori which is resistant to Metronidazole.