Function of Penicillin-binding protein 3 in Streptococcus Faecium

Cefotaxime at concns. around the min. inhibitory concn. (MIC, 5 μM) or below (0.1-1.0 μM) causes transformation of the normal S. faecium cells to bacilliform cells whose length increases with increasing duration of treatment. Septa are initiated but never reach completion. Affinity detn. for cefotaxime binding showed that the antibiotic binds preferentially to the 3 highest mol. wt. penicillin-binding proteins (PBP); PBP-2 and PBP-3 are about 100-fold more sensitive to cefotaxime than PBP-1. It appears, therefore, that PbP-2 and PBP-3 are involved in cell septation. This was confirmed by using cefatoxime in subinhibitory concns. (1 μM). From cell samples collected 30 and 60 min after addn. of cefotaxime, membranes were isolated, labeled with satg. [3H]benzylpenicillin and examd. by fluorog. Under these conditions only PBP-2 and PBP-3 were satd. by cefotaxime. At this stage no distinction could be made between the 2 proteins; either both or 1 of them may be involved in cell division. Cefoxitin produced morphol. alteration of a different nature than cefotaxime. The cefoxitin-treated cells had increased diam. and were slightly elongated. The most striking alteration was the frequent presence of conical poles contrasting with round poles obsd. in control cells. The morphol. alteration obsd. in cefoxitin-treated cells could be attributed to the inhibition of the function of PBP-1, PBP-2, or PBP-3. Elongated cells similar to those obtained with cefotaxime were not found with cefoxitin at concns. sufficient to sat. PBP-2. The main difference between cefotaxime- and cefoxitin-treated cells is that PBP-3 is satd. by cefotaxime but not altered at all by cefoxitin. Thus, septation inhibition must be due to the interaction of cefotaxime with PBP-3