Serratia grimesii are non-pathogenic bacteria capable, however, to invade eukaryotic cells provided that they synthesize intracellular metalloprotease grimelysin (Bozhokina et al.  Cell. Biol. Int. 35: 111–118). To elucidate how invasion of grimelysin containing bacteria depends on physiological state of host cells, we studied the effect of N-acetylcysteine (NAC) on susceptibility of HeLa cells to invasion by the wild-type S. grimesii and recombinant E. coli expressing grimelysin gene. Incubation of HeLa cells with 10 mM NAC resulted in changes of cell morphology and disassembly of actin cytoskeleton that were reversed when NAC was removed from the culture medium. Both in the presence of NAC and upon its removal, the entry of grimelysin producing bacteria increased by a factor of 1.5–2 and 3–3.5 for wild-type S. grimesii and recombinant E. coli, respectively. This effect does not correlate with cytoskeleton rearrangements but may be due to the NAC-induced up-regulation of cell surface receptors playing a role in cell adhesion and cell–cell junctions. A twofold difference in the efficiency of S. grimesii and recombinant E. coli to enter the NAC-treated cells suggests that the entry of the wild-type and recombinant bacteria occurs via different receptors which activity is differently affected by NAC. J. Cell. Biochem. 114: 1568–1574, 2013. © 2013 Wiley Periodicals, Inc.