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SUMMARYBacillus cereus organisms growing exponentially have a stable length distribution. This length distribution can be analysed by the method described to give the mean rate of increase in length of organisms a t any given length. The validity of the method was confirmed by observing the growth of clones of B. cereus in the culture chamber. Both methods showed that the rate of increase in length increased as the organisms got longer, and that there was no hesitation before or after division. Possible applications of this general method to other parameters that can be measured in samples of bacteria taken from stable populations are suggested.

NoviCK, RICHARD P. (The Rockefeller Institute, New York, N.Y.), AND MARK 11. RICHMOND. Nature and interactions of the genetic elements governing penicillinase synthesis in Staphylococcus aureus. J. Bacteriol. 90:467-480. 1965.-It has been found previously that penicillinase-producing strains of Staphylococcus aureus each harbor an extrachromosomal element, or plasmid, which apparently carries all the genetic information necessary for penicillinase synthesis. These plasmids behave in a manner consistent with their being small chromosomelike structures in that they comprise linkage groups containing several markers and in that they undergo such genetic events as mutation, segregation, and recombination. There is currently no evidence for conjugal cell-cell transfer of the plasmids or for a state of stable integration into the staphylococcal chromosome. A certain amount of variability has been encountered among the penicillinase plasmids harbored by different staphylococcal strains. It has been found that: (i) there are at least three molecular variants of the enzyme itself; (ii) most, but not all, of the penicillinase plasmids carry a genetic determinant of resistance to mercuric ion; (iii) plasmids carried by a very small number of the strains bear a determinant of resistance to erythromycin; (iv) the plasmids determine the fraction of penicillinase excreted into the medium during growth, and this also varies from strain to strain. The penicillinase plasmids appear to behave as integral genetic structures. The entire known linkage group is transduced intact, and is occasionally lost completely as a spontaneous occurrence during the growth of the organisms. Rarely, the plasmid markers dissociate during transduction, resulting in transduced clones which have inherited only a part of the plasmid linkage group. Similarly, dissociation occurs as a spontaneous event during normal growth, also resulting in rare clones which appear to have lost one or more but not all of the plasmid markers. When crosses are performed between two plasmid-harboring strains, a plasmid heterozygote is formed. In most cases, this persists for only one or a few cell divisions before segregating, with or without the formation of recombinant plasmids. In two instances thus far observed, the heterozygote persists as a stable plasmid heterodiploid, in which the continued presence of both plasmids can be readily demonstrated. The fate of the heterozygote, i.e., early segregation or persistence as a heterodiploid, depends upon which particular pair of plasmids is involved in the cross. This observation has led to the hypothesis of a plasmid-linked determinant of plasmid compatibility. A pair of plasmids is considered compatible if it can form a stable heterodiploid, incompatible if it cannot. Evidence has previously been presented that Briefly, the evidence is as follows. (i) The penithe genetic determinant of penicillinase formation cillinase determinant is lost freuqently (,10-3 in Staphylococcus aureus is incorporated in an per cell per generatio...

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RP1, a group of genes specifying resistance to carbenicillin, neomycin, kanamycin, and tetracycline and originating in a strain of Pseudomonas aeruginosa , was freely transmissible between strains of P. aeruginosa, Escherichia coli , and Proteus mirabilis . Acquisition of the multiple drug resistance specified by RP1 by these strains was accompanied by acquisition of an extrachromosomal satellite of covalently closed circular deoxyribonucleic acid of molecular weight about 40 million daltons and of buoyant density 1.719 g/cm 3 (60% guanine plus cytosine).

1. Three variants of staphylococcal exopenicillinase (types A, B and C) can be distinguished on chemical, enzymological and immunological grounds. 2. Enzyme type A has a higher specific activity than that of type B, but has a similar combination affinity with anti-(exopenicillinase type A) serum. 3. Enzyme types A and C have a similar specific activity, but enzyme type C has a lower combination affinity for anti-(exopenicillinase type A) serum than has enzyme type A. 4. The sedimentation coefficients and amino acid analyses of the three enzyme types are similar. 5. All three enzyme types have small but significant differences in kinetics of action when hydrolysing benzylpenicillin, methicillin, cloxacillin and cephalosporin C. 6. Peptide maps, obtained from enzyme types A and C after digestion with trypsin, show that these two variants probably differ in the nature of only a very few amino acid residues. 7. Enzyme type B seems to be confined to staphylococci that are members of staphylococcal phage group II. Enzyme types A and C are produced by staphylococci that are members either of phage group I or III, but never group II. 8. The low specific enzyme activity and affinity of enzyme type B towards all penicillins tested suggest that this enzyme type has a lower ;efficiency' in hydrolysing penicillin and therefore in protecting bacteria from the action of penicillin. This could account for the low incidence among ;hospital staphylococci' of penicillin-resistant staphylococci that are members of phage group II.

1. The penicillinase (beta-lactamase) from Escherichia coli strain TEM has been purified and its activity against a range of penicillin and cephalosporin derivatives measured. 2. The enzyme shows little resemblance to penicillinases from Bacillus cereus, Bacillus licheniformis and Staphylococcus aureus. 3. The molecular weight of the enzyme is 16700+/-5%, which is about half the value obtained for other penicillinases. 4. The enzyme is most similar in properties to a crude preparation of a penicillinase from Klebsiella (Aerobacter) aerogenes, but clearly different from crude enzyme preparations from other strains of E. coli. 5. Since penicillinase synthesis in E. coli strain TEM is mediated by an R-factor known to infect many other species of Enterobacteriaceae, the appearance of similar enzymes in other Gramnegative species is not surprising.

Escherichia coli UB1005 and two mutants of this strain (DC2 and DC3) have been used to assess indirectly the relative ability of various β-lactam antibiotics to penetrate the outer layers of E. coli . Benzylpenicillin, ampicillin, methicillin, cloxacillin, cephaloridine, cephalothin, and cephalexin have been examined. The results confirm those obtained with other methods and show that, among the compounds studied here, cephalosporins seem to penetrate more readily than penicillins.