Biological properties of alpha-toxin mutants of Staphylococcus aureus

Pathogenicity in mice of Staphylococcus aureus mutants deficient in exoprotein synthesis

Twelve mutants were isolated from a Staphylococcus aureus strain derived from bovine mastitis after mutagenesis by ultraviolet light. These mutants were found to be deficient for several characteristics such as production of most exoproteins and had altered phage type and/or colonial morphology in serum-soft agar medium. They also differed in virulence when assayed in mice by intraperitoneal administration; the ratio of the LD50 of the mutants vs. that of the parental strain ranged from 1 to 123. The different virulence of the mutants could not be associated with lack of production of exoproteins or altered colonial morphology. On the other hand, a clear correlation was evidenced between lowered virulence and slower growth rate at 37 degrees C. Three mutants were assayed in the mouse mastitis model. One of them, which was about 40 times less virulent when assayed by intraperitoneal administration, induced a histopathological lesion similar to that produced by the parent strain; the other two mutants, which were about 70 to 120 times less virulent by intraperitoneal administration, induced only a very slight lesion. Mice were vaccinated by the intraperitoneal route with two of the less virulent mutants; the LD50 in the vaccinated mice that were challenged with the parental strain increased 11 to 14 times compared with that for the unvaccinated mice.

Chromosomal map location of the alpha-hemolysin structural gene in Staphylococcus aureus NCTC 8325

The alpha-hemolysin structural gene, hly+, previously cloned, insertionally inactivated, and introduced into the chromosome by allele replacement (M.O. O'Reilly, J.C.S. De Azavedo, S. Kennedy, and T.J. Foster, Microb. Pathog. 1:125-138, 1986), was shown by protoplast fusion and transformation to be in the gene order purC-hly-uraB-omega[chr::Tn916]1101-thrB on the chromosome of Staphylococcus aureus NCTC 8325. This location is clearly distinct from that of the agr determinant, a regulatory gene affecting several extracellular proteins, including alpha-hemolysin, located between tmn and ilv.

Identification of a chromosomal determinant of alpha-toxin production in Staphylococcus aureus

Production of alpha-toxin (the Hla+ phenotype, controlled by the Hla gene and scored as alpha-hemolytic activity) is a property of some isolates of Staphylococcus aureus NCTC 8325 and not of others. Genetic transformation between strains differing in the Hla phenotype revealed that the hla+ gene resides in the following sequence: purB110-bla+-hla+-ilv-129-pig-131; previously, the enterotoxin A (entA) gene of strain S-6 was shown to map very close to hla+. The hla+ mutations occurring naturally in strain Ps6 and after various mutagenic treatments in strains 8325 and 233 also mapped between bla+ and ilv-129. Among the isolates of strain 8325, the Hla+ phenotype was always associated with fibrinolytic activity, whereas Hla- isolates were non-fibrinolytic. This relationship was also observed among transformants selected for their Hla+ or Hla- phenotypes. The failure of Hla- strains and mutants to revert to hla+ at detectable frequencies, the instability of the Hla+ phenotype, and the previously observed pattern of recombination of the hla+ and entA+ determinants lend support to the view that hla+ may reside on a transposon; according to this view, Hla- mutants have lost the hla+-bearing transposon. It remains unclear whether hla+ is the structural gene for alpha-toxin.

Spontaneous alpha-toxin mutants of Staphylococcus aureus

Prolonged cultivation of strain Wood 46 in fluid cultures resulted in a selection of mutants with low or no haemolytic activity. In one group of mutants, four out of five strains showed no production of alpha-toxin when examined by polyacrylamide gel electrophoresis and by double diffusion in agar. Two major extracellular proteins which have been identified by other methods as degradation products of alpha-toxin were also absent. The absence of alpha-toxin did not affect growth in fluid or solid media. Fibrinolysin was produced by these mutants but at a much lower rate than by the wild type. A second group of mutants was characterized by a slow rate of growth on rabbit blood agar and showed a heterogeneous extracellular protein pattern. These mutants had a high growth rate in fluid medium consisting of acid hydrolysed proteins. Production of fibrinolysin was absent or low in three out of four mutants in the second group. The slow growth and low production of alpha-haemolysin in rabbit blood agar probably was caused by deficient extracellular proteolytic activity of the mutants.

Iodination of a tyrosyl residue in staphylococcal alpha-toxin

Iodination of staphylococcal alpha-toxin by the lactoperoxidase method resulted in the maximal incorporation of about 2.5 atoms of iodine per molecule of alpha-toxin. The iodination primarily involved a single tyrosine residue as shown by analysis of both cyanogen bromide and tryptic peptides. Iodination at a level of 1.2 iodine atoms per alpha-toxin molecule led to a dramatic decrease in the hemolytic and lethal activities, although no decrease in the binding of iodinated toxin to rabbit erythrocytes was observed (Cassidy and Harshman (1976), Biochemistry, the following paper in this issue). Monoiodinated alpha-toxin was found to have 15% of the specific hemolytic activity of native alpha-toxin. Incubation of rabbit erythrocytes with iodinated alpha-toxin led to a significant protection from the hemolytic activity of native alpha-toxin added later. The results show the modification of a single unique tyrosyl residue in alpha-toxin permits the resolution of alpha-toxin's biological activities from its cell binding activity.

The role of humoral immunity and acute inflammation in protection against staphyloccocal dermonecrosis

Mice were protected against the dermonecrotic effects of Staphylococcus aureus by previous infection with either coagulase-positive or coagulase-negative strains or by immunization with alpha-toxin. Passive protection was conferred by serum from previously infected mice or by alpha-antitoxin. While only some of these methods were associated with circulating alpha-antitoxin, in all cases there was a brisk early inflammatory response to infection. Furthermore, if the capacity of well immunized mice to mount such a response was removed, they were no longer protected against dermonecrosis. Conversely, non-immune mice developed little or no necrosis if the staphylococci were injected into areas of preexisting non-specific acute inflammation whether these had been produced chemically or immunologically. It is suggested that in this model of local infection with S. aureus an early inflammatory response, however provoked, is the major protective factor. Though specific neutralizing actions of antibodies are not excluded, the most important result of antibody-antigen reaction is to cause local inflammation by some form of immediate hyersensitivity.

Pleiotropic alteration of activities of several toxins and enzymes in mutants of Staphylococcus aureus

Pleiotropic alteration of several genetic characters including toxin production was quantitatively shown with a strain of Staphylococcus aureus of phage type 80, 81 which had been given a very specific genetic marker (temperature sensitivity of mannitol fermentation) to avoid confusion by contamination. Thus, alpha-hemolysin hyperproducers obtained by N-methyl-N'-nitro-N-nitrosoguanidine (NTG) mutagenesis were very often hyperproducers of DNase, coagulase, and protease. Their colonies were less yellow than the parent. DNase hyperproducers obtained after NTG mutagenesis were also often hyperproducers of alpha-hemolysin, coagulase, and protease, with colonies less yellow than the parent. Almost all of the revertants obtained by mutagenesis with ethyl methane sulfonate with respect to alpha-hemolysin or DNase were shown to have simultaneously become hypoproducers of alpha-hemolysin, DNase, and protease. Since the pleiotropic alteration of multiple functions was thus quantitatively confirmed, the mechanism underlying this phenomenon should probably be related to a regulatory mechanism common to them.

Protein A mutants of Staphylococcus aureus

Staphylococcus aureus Cowan I was exposed to nitrosoguanidine or ethyl-methanesulfonate, and survivors were screened on nutrient agar plates containing rabbit anti-protein A serum for loss of protein A production. More than half of all protein A-deficient mutants also lacked nuclease, coagulase, alpha hemolysin, fibrinolysin, mannitol utilization, and the phage-type pattern. Mutants with a spectrum of these properties were also isolated. Induced or spontaneous reversions of the mutants were observed. The properties of the protein A-deficient mutants suggest that synthesis or release (or both) of a number of extracellular products of S. aureus is controlled by a common regulatory mechanism.

Isolation of mutants of Staphylococcus aureus lacking extracellular nuclease activity

Suspensions of Staphylococcus aureus Foggi strain were exposed to nitrosoguanidine and screened on deoxyribonucleic acid-acridine orange-agar plates for loss of extracellular nuclease activity. All nuclease-deficient mutants lacked cross-reacting material when tested with antinuclease antibody. Coagulase and beta-hemolysin were also lost in nuclease-deficient mutants, and all three enzyme functions were regained together upon reversion with ethyl methane sulfonate. These observations are consistent with the suggestion that the synthesis or the release, or both the synthesis and release, of certain extracellular enzymes may be subject to coordinated control mechanisms.

Genetic recombination between alpha-toxin mutants of Staphylococcus aureus

McClatchy, J. K. (The University of Texas Southwestern Medical School, Dallas), and E. D. Rosenblum. Genetic recombination between alpha-toxin mutants of Staphylococcus aureus. J. Bacteriol. 92:580-583. 1966.-A demonstration of genetic recombination between Staphylococcus aureus nonhemolytic mutants was attempted by means of transduction. The results of two-point reciprocal transductions placed the mutants into two genetic groups. Recombination within each group was not detectable within the limits of the method, but hemolytic recombinants were obtained in transductional crosses when donor and recipient were from different groups. At least two genetic loci are therefore involved in alpha-toxin production. The 11 mutants of group II were fibrinolysin-negative. The recombinants were always found to be restored to fibrinolysin production as well as to alpha-toxin production. These data suggest the existence of a pleiotropic gene simultaneously affecting the synthesis of both alpha toxin and fibrinolysin. The nine mutants of group I were fibrinolysin-positive. Group I members are postulated to be alpha-toxin structural mutants. Three mutants were also negative for bound coagulase, but no linkage was observed between the locus controlling bound coagulase and the loci for either fibrinolysin or alpha-toxin production.