The potent heat-labile bactericidal property of fresh mammalian defibrinated blood or serum in vitro has been known since the studies of Nuttall (1) and Buchner (2) over 70 years ago. By 1928, it had been shown that serum components very like those of complement were necessary for this action (3). Although a number of investigators in the fields of bacteriology and immunology have interested themselves in this bactericidal power of fresh serum, little notice has been taken of it in clinical medicine, probably, as noted elsewhere (4), because it has been believed to be of doubtful effect in vivo.Interest has been revived in the bactericidal activity of serum by the work of Pillemer, Wardlaw and associates (5, 6). They described a normally occurring serum protein of large molecular weight which they named properdin. They attributed the bactericidal property of serum, in part at least, to the presence of properdin, which acted with the four components of complement in the presence of magnesium to kill the bacteria.The findings of Wardlaw and Pillemer (6) confirmed those of Mackie and Finkelstein (7) that the bacterial strains sensitive to the effect of serum are principally from species of gram-negative bacilli and that the degree of sensitivity is a characteristic of the strain rather than of the genus. However, whereas Wardlaw and Pillemer have described the action of the properdin system as being nonspecific and unrelated to antibody, Mackie and Finkelstein presented data showing that the absorption of a serum in the cold by a sensitive strain resulted in the removal of the killing effect only for that strain, leaving the effect

Antibiotic resistances of two sets of SdlnIOnelld typhimurium rfd transductants (along with those of their smooth pyrE+ and c p E + sister transductants) were measured. One set was derived from a pyrE smooth L T~ parent and the other from a cysE smooth LT7 parent. Results showed that strains with defects at the rfa(R-res-2) level and deeper were more susceptible to bacitracin, novobiocin and polymyxin. Those with defects at the rfdG level or deeper were in addition more sensitive to vancomycin, erythromycin, oxacillin and nafcillin. At these levels the presence or abse.nce of galactose I or glucose I from the lipopolysaccharide core made a considerable difference. A heptose-less rfaE mutant was the most sensitive of the strains tested to the above named antibiotics. Strains with rfa lesions at several levels of defect showed slight increases in resistances to tetracycline, cephalothin, ampicillin and penicillin.One would expect strains with gdlE mutations to be similar to rfa(R-res-2) strains and ' those with galU mutations to be similar to rfaG strains if the core defects accounted for the differing antibiotic resistances. They proved to be so except that the galE and gdlu strains in the S. typhimurium FIRN line were as resistant to novobiocin as were smooth strains. The results are interpreted in respect to Nikaido's hypothesis that hydrophilic antibiotics with molecular weights of less than about 650 can gain access to the periplasmic space through protein-lined, water-filled pores and that hydrophobic ones can gain access in deep rough strains when phospholipid patches appear on the surface due to absence of polysaccharides and proteins.

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S U M M A R YSeveral mutants obtained from smooth SalmoneIla typhimurium strains by selection for resistance to Felix 0 (FO) phage [whose receptor site includes the N-acetylglucosamine branch of the lipopolysaccharide (LPS) core] were smooth in cultural properties, antigenic character and phage sensitivity pattern (except for their FO resistance). However, the affected genes of several such 'FOR' (FOresistant) mutants were shown by transduction to map in the short cysE-pyrE segment, which includes nearly all known rfa genes responsible for synthesis of LPS core. All of seven FOR mutants differed from their parents, and resembled rfa mutants with defects in the deeper part of the LPS core, by increased sensitivity to various antibiotics. One FOR mutant was non-virulent (LD,, > 107, compared with < IOO for its parent); L T~ derivatives given this FOR gene by co-transduction with cysE+ were likewise non-virulent. It is inferred that FOR mutations affect the assembly of the inner part of the LPS core, perhaps causing incomplete blocks in glycosyl transferase reactions. I N T R O D U C T I O NFelix 0 phage (hereinafter called FO) attacks nearly all smooth strains of Salmonella, whatever their 0 antigen, but is active on very few strains of Escherichia coli, etc. (Kallings, 1967). In Salmonella typhimurium it attacks not only smooth strains but also some nonsmooth mutants: those unable to transfer 0 chains from their site of synthesis to the 'complete' lipopolysaccharide (LPS) core, through mutation at rfaL or rfbT; the semirough (SR) class rfc, deficient in polymerization of 0 repeating units; and classes rfb and ymi, unable to synthesize 0 repeating units (Wilkinson, Gemski & Stocker, 1972). Phage FO, however, does not attack rfa mutants of various classes in which synthesis of the LPS core is defective. Lindberg and his colleagues (for review see Lindberg, 1973), from host range and from measurements of rates of adsorption by whole bacteria and of neutralization by isolated LPS, infer that the N-acetylglucosamine side-branch which is attached to the glucose I1 unit of the complete LPS core (Fig. I ) is an essential part of the adsorption site of FO phage. Most mutants selected from smooth strains by application of FO phage are of type [fa, with various sorts of defects in LPS core structure, and are recognizable as such by sensitivity to different combinations of rough-specific phages and/or to bile salts and by total or partial loss of 'smooth' cultural characters, 0 antigen(s) and sensitivity to smoothspecific phages (Wilkinson et a/. 1972). We here describe FO-resistant mutants of smooth * Present address :

SUMMARYSerum-sensitive mutants have been derived from serum-resistant smooth virulent Salmonella typhimurium and S. enteritidis strains by selection for resistance to cephalosporin or penicillin. Chemical analyses of the lipopolysaccharides of these mutants reveal that they belong to at least three different rough or semi-rough classes. Partial or total loss from the lipopolysaccharide of the sugars responsible for 0 antigenicity resulted in loss of virulence, as well as increased sensitivity to the bactericidal effect of antibody plus complement. However, such loss is not necessary for serum sensitivity because two serum-sensitive mutants possessed lipopolysaccharides indistinguishable from the smooth serum-resistant parents and were nearly as virulent.