Chromogenic detection of antigen in bacteriophage plaques: a microplaque method applicable to large-scale screening

We have developed a simple and rapid (24 h) enzyme-linked immuno-detection method to screen for rare antigen-positive phage among large numbers of antigen-negative ones. Horse-radish peroxidase-antibody conjugate, incorporated into the soft agar layer of a plaque assay system, is precipitated locally by antigen produced during plaque formation, and is detected by standard chromogenic methods. The method has been used to screen plaques of bacteriophage beta tox+ for the presence of diphtheria toxin and related cross-reacting material. When phage were plated on very dense bacterial lawns, they formed minute plaques (microplaques). Because of the high local concentration of antigen generated by lysis of the dense lawn, the microplaques gave more intense chromogenic signals than larger plaques formed on less dense Corynebacterium diphtheriae lawns. Thus, antigen-positive microplaques could be easily recognized even in the presence of very large numbers of antigen-negatives. In a reconstruction experiment, small numbers of antigen-positive phage were detected with high efficiency (greater than 75%) against a background of 3.8 X 10(4) antigen-negatives/cm2 of agar surface (equivalent to 2.4 X 10(6) plaques/9 cm petri plate). This screening method should facilitate isolation of phage mutants affecting production of given antigens and may be of particular value in detecting specific genes cloned into phage vectors.

Appendix: a model of plaque formation

Equations describing plaque formation in soft agar have been based on certain simplifying assumptions, for which data are presented. The derived equations permit one to calculate (i) average plaque size as a function of the initial density of indicator cells (Do), (ii) the number of cells lysed per plaque as a function of Do, and (iii) the cumulative number of cells lysed at various stages of plaque development. The calculated values agree well with those determined experimentally.

Toxicity of diphtheria toxin-related proteins produced by suppression of nonsense mutations

The Mr = 62,000 diphtheria toxin-related proteins produced from the suppression of nonsense mutations within the tox gene of corynephage beta were purified by affinity chromatography. Except for the toxin 111-sup2-62, the Mr = 62,000 polypeptides were found to have the same specific toxicity as does wild type toxin. 111-sup2-62 was found to have a prolonged lag period prior to the onset of inhibition of protein synthesis and ADP-ribosylation of elongation factor 2. 111-sup2-62 differs from wild type toxin by an amino acid substitution at a site approximatley 47,000 daltons from the NH2 terminus. The data presented provide genetic support for the Boquet-Pappenheimer model (Boquet, P., and Pappenheimer, A. M. Jr (1978) J. Biol. Chem. 251, 5770-5778) of fragment A translocation into the eukaryotic cell cytosol.

Plasmids in Corynebacterium diphtheriae and diphtheroids mediating erythromycin resistance

Plasmids were isolated from erythromycin-resistant Corynebacterium diphtheriae and skin coryneforms. Six erythromycin-resistant C. diphtheriae strains, isolated from cutaneous lesions, all contained a 9.5-megadalton (Mdal) plasmid. Loss of resistance was associated with the deletion of a 1-Mdal segment from the plasmid or, less frequently, with loss of the plasmid. Two erythromycin-resistant diphtheroids were isolated from similar skin lesions. One contained a 38-Mdal plasmid that was lost in the conversion to erythromycin susceptibiliy. The other diphtheroid contained a 30- and a 14-Mdal plasmid. Erythromycin-susceptible derivatives of this strain were not recovered. Restriction enzyme analysis indicated that the 9.5-Mdal plasmids in the C. diphtheriae strains are very similar, if not identical, and that each of the deleted plasmids has lost the same 1-Mdal segment. However, the restriction patterns of the plasmids in the two diphtheroids are not closely related to each other nor to the plasmids in the C. diphtheriae strains.

Immunogenic correlation between cross-reacting material (CRM197) produced by a mutant of Corynebacterium diphtheriae and diphtheria toxoid

The in vivo immunizing potency of diphtheria toxoid and formalin-treated corss-reacting material (CRM197, a nontoxic mutant protein) was compared in guinea pigs. Major antigenic differences between the two untreated proteins were also tested in rats. The results showed that diphtheria toxoid and CRM197 were equally effective immunogens, but only if the latter was treated with formalin in the same concentration (0.7% vol/vol) was that of the toxoid. Formalin treatment rendered the antigens more resistant to enzymatic proteolysis by trypsin in vitro.

Isolation and characterization of extragenic suppressor strains of Corynebacterium diphtheriae

The isolation and characterization of two different nonsense suppressor strains of Corynebacterium diphtheriae C7 sup+(-)tox- are described. Appropriate lysogens of these strains with corynephage beta, carrying known class II tox premature polypeptide chain termination mutations [C7sup-1(betatox-30) and C7sup-2(betatox-45)], each produce a 62,000-dalton polypeptide with nicotinamide adenine dinucleotide: elongation factor-2 adenosine diphosphate ribosyltransferase activity in addition to a chain-terminated polypeptide of 30,000 or 45,000 daltons, respectively. In addition, purified protein of 62,000 daltons, resulting from the suppression of the nonsense mutations tox-30 and tox-45, will react with antisera purified against the terminal 17,000 daltons of the toxin molecule and are immunologically identical to toxin by radial immunodiffusion. The suppression pattern of lysogenic derivatives of C7sup-1(-)tox- and C7sup-2(-)tox- with other class II and III mutants of corynephage beta was determined.

Three variants of Corynebacterium diphtheriae subsp. mitis (Belfanti) isolated from a throat specimen

Three variants of a Corynebacterium diphtheriae subsp. mitis strain belfanti, two toxigenic (phage types 7 and 11) and one nontoxigenic (phage type 7), were simultaneously isolated from the throat of an adult.

Control of corynebacteriophage reproduction by heteroimmune repression

Corynebacteriophages beta and gamma are closely related but heteroimmune; hence, gamma reproduces in C7(beta). A series of gamma mutants, designated gamma-bin (beta-inhibited), has been isolated. They reproduce in only 2 to 14% of infected C7(beta) cells, and, as a result, plaque with an efficiency of 10(-4) to 10(-5) on this strain. The proportion of C7(beta) cells in which gamma-bin phage can replicate is increased to 30 to 80% when immunity is lifted by UV induction of C7(beta) or by heat induction of C7(beta-tsr3). The gamma-bin mutants carry out a normal vegetative or lysogenic cycle in strain C7 and thus do not appear to be defective in any essential phage function. Infection of C7(beta) by gamma-bin results in cell killing whether the infection is productive or nonproductive. The data support the hypothesis that inhibition of gamma-bin is due to the direct or indirect action of a beta prophage gene. The simplest hypothesis is that gamma-bin phages have sustained mutations in an operator site and that beta repressor now combines with the mutated operator to inhibit normal replication in a significant proportion of infected cells.

Evidence that the regulation of diphtheria toxin production is directed at the level of transcription

It has been known for several decades that iron inhibits the production of diphtheria toxin by Corynebacterium diphtheriae by preventing expression at maximal levels. We examined the inhibition kinetics of toxin production after the addition of either iron or rifampin to iron-limited cultures of C7 (betatox+). Iron-mediated inhibition of toxin production was found to be linear within the range of 16 nM to 16 micron. The inhibition kinetics following the addition of iron or rifampin was almost identical. [3H]RNA extracted from iron-limited toxigenic C. diphtheriae was found to hybridize to a greater extent to corynephage beta DNA than either [3H]RNA extracted from toxigenic C. diphtheriae before the onset of toxin production or [3H]RNA extracted from nonlysogenic, nontoxigenic C. diphtheriae.

Isolation of the L-phase variant from toxigenic Corynebacterium diphtheriae C7(beta)

The L-phase variant was isolated from lysogenic strain C7(beta) of Corynebacterium diphtheriae by the penicillin disk tecnnique on an agar medium containing 3.7% brain heart infusion, 3% NaCl, 1.25% Grycine, 1% Noble agar, and 10% horse serum. The L-phase formed a "fried egg" colony; it was resistant to antibiotics that inhibit biosynthesis of bacterial cell walls but was susceptible to antibiotics that act on the protein-synthesizing system. After several transfers of the L-phase initially isolated, the L-phase of strain C7(beta) became stable, forming a typical L-phase colony on the agar even without penicillin. The L-phase variant retained lysogenicity and toxigenicity. The toxin production by this L-phase variant was, like that by the bacterial phase, inhibited by excess iron.

[The species C. diphtheriae]

Corynebacteria (C. diphtheriae var. gravis, mitis, minimus, intermedius, C. belfanti, C. ovis, C. ulceraus) producing diphtheria toxin are found as variants sharply differing from one another morphologically (from long branching rods to coccobacilli), culturally, biochemically and by other signs. The toxin synthesis in them was determined by genetically heterogeneous prophages tox+; tox+ virus exchange is possible between the corynebacteria. Since production of specific toxin, the cardinal sign of C. diphtheriae species, is connected with the genoms of genetically heterogeneous viruses tox+, the circle of whose hosts is very wide, it can be supposed that there is no genetically homogeneous taxonomic category "diphtheriae species". Several species of corynebacteria--lysogenized (toxigenic) or capable of being lysogenized (nontoxigenic) with tox+ viruses are included into the composition of the corynebacteria groups which medicine considers as "species" for over 80 years. A possiblity of becoming a diphtheria causative agent is determined by the capacity of corynebacteria to become infected with the circle of viruses tox+ the DNA of which can become stably integrated with the gene of this bacterium. The mentioned approach to taxonomy is possibly applicable to microorganisms in which the pathogenicity signs (the toxin) were determined, as in C. diphtheriae, in the virus genoms (Bac. botulinus).

Isolation from corynebacterium diphtheriae C7(beta) of bacterial mutants that produce toxin in medium with excess iron

Five mutants that produce toxin in medium with excess iron were isolated from strain C7(beta). The iron content of bacteria grown on this medium was considerably higher than that of C7(beta) cells grown in medium containing the minimum amount of iron needed to inhibit toxin production. When the nonlysogenic, nontoxinogenic strain C7(-) was lysogenized with phages from each of the mutants, toxin production by all of the resulting lysogens, like that by parent strain C7(beta), ceased upon iron addition. When the mutants were superinfected with beta45 phage, both toxin and CRM45 were produced in medium with excess iron. One of the mutant strains lost its prophage as a result of treatment with ultraviolet light. When the cured strain was lysogenized with phage carrying a mutation in the tox structural gene, the lysogen produced the mutant protein at the maximum rate in medium with excess iron. These findings show that the mutant strains are not phage mutants, but are bacterial host mutants, and that a host factor(s) is involved in the inhibition of toxin production by iron.