ABORTIVE INTESTINAL INFECTION WITH AN ESCHERICHIA COLI-SHIGELLA FLEXNERI HYBRID STRAIN

Role of the virulence plasmid in acid resistance of Shigella flexneri

Virulence plasmid (VP) acquisition was a key step in the evolution of Shigella from a non-pathogenic Escherichia coli ancestor to a pathogenic genus. In addition, the co-evolution and co-ordination of chromosomes and VPs was also a very important step in the evolutionary process. To investigate the cross-talk between VPs and bacterial chromosomes, we analyzed the expression profiles of protein complexes and protein monomers in three wild-type Shigella flexneri strains and their corresponding VP deletion mutants. A non-pathogenic wild-type E. coli strain and mutant E. coli strains harboring three Shigella VPs were also analyzed. Comparisons showed that the expression of chromosome-encoded proteins GadA/B and AtpA/D, which are associated with intracellular proton flow and pH tuning of bacterial cells, was significantly altered following acquisition or deletion of the VP. The acid tolerance of the above strains was also compared, and the results confirmed that the presence of the VP reduced the bacterial survival rate in extremely acidic environments, such as that in the host stomach. These results further our understanding of the evolution from non-pathogenic E. coli to Shigella, and highlight the importance of co-ordination between heterologous genes and the host chromosome in the evolution of bacterial species.

Genetic Transfer of Shigella flexneri Antigens to Escherichia coli K-12

The genes controlling synthesis of Shigella flexneri group- and type-specific antigens were transferred to Escherichia coli K-12 recipients by conjugation with an S. flexneri Hfr. After mating E. coli with an Hfr strain of S. flexneri 2a and selecting for his(+) recombinants, a high proportion of the E. coli hybrids agglutinated in S. flexneri grouping serum. None of these hybrids expressed S. flexneri type-specific antigen II. When an E. coli his(+) hybrid possessing the S. flexneri group antigen was remated with the same Hfr with selection for pro(+) hybrids, a high proportion now expressed the type-specific antigen as well as the previously inherited group antigen. If such crosses were performed in reverse order (i.e., pro(+) followed by his(+) selection), a different pattern of serological behavior was observed. None of the pro(+) hybrids showed the type-specific antigen. Subsequent mating for his(+) resulted in hybrids with both the group- and type-specific antigens. These results show that genes controlling the synthesis of S. flexneri group antigen (linked to the his locus) and type-specific antigen (linked to the pro locus) are widely separated on the chromosome. Expression of the type-specific antigen II depends on the presence of the group antigen.

Infection of Cultured Mouse Macrophages with Shigella flexneri

Virulent, avirulent, and attenuated hybrid strains of Shigella flexneri 2a are equally susceptible to phagocytosis by cultured mouse peritoneal macrophages. The virulent strain is highly lethal for the macrophages, whereas the avirulent is not and is killed. The attenuated hybrid strain is intermediate in its lethality. Comparable results were obtained by using virulent and avirulent S. flexneri 1b, 3, and 5. Destruction of macrophages occurs shortly after infection, suggesting virulent strains may possess a toxic component. The relationship of the ability to kill macrophages with multiplication of virulent shigellae in mucosal tissue is discussed.

A Chromosomal Locus Which Controls the Ability of Shigella flexneri to Evoke Keratoconjunctivitis

The primary step in the pathogenesis of bacillary dysentery is the penetration of intestinal epithelial cells by shigellae. Lacking this capacity, Shigella flexneri becomes avirulent. By means of intergeneric conjugation between various Escherichia coli K-12 Hfr strains and S. flexneri 2a virulent recipients and by reciprocal transduction analysis with phage P1 vir, we established a locus on the genome of S. flexneri 2a which is necessary for the ability of this strain to penetrate epithelial cells as measured by the Sereney test for keratoconjunctivitis. This locus, termed kcpA (in reference to its involvement in provoking keratoconjunctivitis), has been positioned between the lac and gal chromosomal markers and is contransducible with the purE allele.

Virulence of R-Factor-Bearing Salmonella typhimurium

Antibiotic-resistant recombinants obtained from mating antibiotic-resistant Escherichia coli with virulent, antibiotic-sensitive Salmonella typhimurium are generally avirulent. After 32 consecutive transfers, two of four avirulent recombinants regained their virulence without loss of episome-mediated resistance.

Studies on the pathogenesis of enteric infections caused by invasive bacteria

Salmonellae, shigellae and some Escherichia coli must invade the intestinal epithelial cell and multiply within the mucosa to cause disease. Although the bacterial cell most likely possesses several properties essential to this invasive ability, the nature of the cell envelope complex is at present the only characteristic which has been implicated in this process. While a number of pathophysiological events result from invasion, some of our recent efforts have concerned the site and mechanism of intestinal fluid loss in salmonellosis and shigellosis. In both these disorders, bacterial invasion of the colonic mucosa, associated with an acute inflammatory reaction and mucosal damage, is regularly seen and colonic salt and water transport is abnormal. These defects may account for mild diarrhoea in salmonellosis and the dysenteric stools of shigellosis. However, in salmonella-infected animals with severe watery diarrhoea and in shigella-infected animals with diarrhoea alone or in combination with dysentery, the jejunum is in a net secretory state. This secretion occurs in the absence of bacterial invasion or morphological abnormalities. Thus, the diarrhoea caused by invasive bacteria may result from the inability of the colon to reabsorb the increased volume of fluid entering it from the small intestine. Although colonic mucosal damage is a feature of invasive-type diarrhoeas, the permeability of both the colon and small intestine to small molecules, mannitol and erythritol, is not altered. Thus intestinal fluid loss cannot be ascribed to transudation. In addition, the results of our Ussing chamber experiments, employing salmonella-infected rabbit ileum, reveal that salt and water secretion is an active process. Since secretion occurs in the jejunum in the absence of bacterial invasion, this might suggest the participation of an enterotoxin. Shigella dysenteriae I is the best-studied invasive organism in which an enterotoxin has been found, yet mutant strains which do not invade but retain the ability to elaborate enterotoxin fail to cause disease in either monkeys or man. Thus, the physiological relevance of Shiga enterotoxin and the mechanism of jejunal secretion in these disorders remain unclear. Recent data suggest that invasive enteropathogens, like the enterotoxin-producing bacteria, activate the mucosal adenylate cyclase-cyclic AMP system and that this activation may play a role in intestinal fluid secretion.

Protection of Monkeys Against Experimental Shigellosis with Attenuated Vaccines

Formal, Samuel B. (Walter Reed Army Institute of Research, Washington, D.C.), E. H. LaBrec, Amos Palmer, and Stanley Falkow. Protection of monkeys against experimental shigellosis with attenuated vaccines. J. Bacteriol. 90:63-68. 1965.-Two Shigella flexneri 2a strains of reduced virulence were used as oral vaccines to protect monkeys against experimental challenge. One strain, a spontaneous mutant, had lost its ability to cause disease and was unable to penetrate the intestinal epithelium and reach the lamina propria. The other strain was a hybrid obtained by mating virulent S. flexneri 2a with Escherichia coli. This hybrid strain retained the capacity to penetrate the intestinal epithelium but was not able to maintain itself in the lamina propria. Five oral doses of the nonpenetrating mutant strain were required to render monkeys resistant to experimental challenge, but a single dose of the hybrid strain sufficed to protect the animals. There was some evidence that a degree of specificity was involved in the induced resistance, although neither vaccine evoked a consistent serum antibody or a detectable coproantibody response.

Construction and characterization of bivalent Shigella flexneri 2a vaccine strains SC608(pCFAI) and SC608(pCFAI/LTB) that express antigens from enterotoxigenic Escherichia coli

An invasive strain of Shigella flexneri 2a (SC608) has been developed as a vector for the expression and delivery of heterologous antigens. SC608 is an aspartate semialdehyde dehydrogenase (asd) derivative of SC602 (icsA iuc), a well-characterized live attenuated vaccine strain which has undergone several clinical trials in human volunteers. When administered orally at a single 10(4) (CFU) dose, SC602 is both immunogenic and efficacious against shigellosis. Using asd-based plasmid vectors, we designed SC608 to express the enterotoxigenic Escherichia coli (ETEC) fimbrial subunit CfaB (CFA/I structural subunit) alone or in combination with the E. coli B subunit of heat-labile enterotoxin (LTB). The expression of each heterologous protein in SC608 was verified by immunoblot analysis. Each strain was comparable to the parent strain, SC602, in a HeLa cell invasion assay. After intranasal immunizations of guinea pigs, serum and mucosal immune responses were detected against both Shigella lipopolysaccharide and heterologous ETEC antigens by enzyme-linked immunosorbent assay and ELISPOT analysis. All immunized animals were subsequently protected against a challenge with wild-type S. flexneri 2a in a keratoconjunctivitis Sereny test. Serum antibodies generated against LTB and CfaB demonstrated antitoxin and agglutination activities, respectively. These results suggest that CfaB and LTB expressed in SC608 retain important conformational epitopes that are required for the generation of antibodies that have functional activities. These initial experiments demonstrate that a fully invasive Shigella vaccine strain can be engineered to deliver antigens from other diarrheal pathogens.

AroD deletion attenuates Shigella flexneri strain 2457T and makes it a safe and efficacious oral vaccine in monkeys

The aromatic-dependent live Shigella flexneri 2a vaccine strain SFL1070, with a deleted aroD gene, had a much reduced intracellular growth in HeLa cells compared with its parent strain S. flexneri 2457T. S. flexneri SFL1070 gave no adverse effects in eight Macaca fascicularis monkeys orally vaccinated with four doses of 1 x 10(11) live bacteria within a 5-week period, whereas S. flexneri 2457T caused dysentery in all eight non-vaccinated monkeys. Thus the aromatic dependency rendered S. flexneri SFL1070 significantly attenuated (p = 0.00008). Significant intestinal S. flexneri lipopolysaccharide (LPS)-specific sIgA responses were seen in seven of eight vaccinated monkeys (p < 0.01) after four doses with SFL1070. However, serum IgG or IgA responses to various S. flexneri LPS antigens and the invasion plasmid antigens (Ipa-s) were seen in only four of eight vaccinated monkeys. The serum IgG titre increases against S. flexneri Y and 2a LPS reached significant levels (p < or = 0.05). All but one of the vaccinated monkeys were protected against oral challenge with 1 x 10(10) or 1 x 10(11) live S. flexneri 2457T given 2 weeks after the last vaccination. The protection was highly significant (p = 0.0007) as all non-vaccinated monkeys challenged with equal doses of strain 2457T developed dysentery. Three of them succumbed. Challenge infection of vaccinated monkeys elicited serum IgA and IgG responses to the homologous S. flexneri 2a LPS in three monkeys each (0.005 < or = p < or = 0.025). Serum IgA and IgG responses to the Ipa-s were seen in five and four monkeys each (0.01 < p < or = 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic basis of virulence in Shigella species

Shigella species and enteroinvasive strains of Escherichia coli cause disease by invasion of the colonic epithelium, and this invasive phenotype is mediated by genes carried on 180- to 240-kb plasmids. In addition, at least eight loci on the Shigella chromosome are necessary for full expression of virulence. The products of these genes can be classified as (i) virulence determinants that directly affect the ability of shigellae to survive in the intestinal tissues, e.g., the aerobactin siderophore (iucABCD and iutA), superoxide dismutase (sodB), and somatic antigen expression (rfa and rfb); (ii) cytotoxins that contribute to the severity of disease, e.g., the Shiga toxin (stx) and a putative analog of this toxin (flu); and (iii) regulatory loci that affect the expression of plasmid genes, e.g., ompR-envZ, which mediates response to changes in osmolarity, virR (osmZ), which mediates response to changes in temperature, and kcpA, which affects the translation of the plasmid virG (icsA) gene which is associated with intracellular bacterial mobility and intracellular bacterial spread. A single plasmid regulatory gene (virF) controls a virulence-associated plasmid regulon including virG (icsA) and two invasion-related loci, i.e., (i) ipaABCD, encoding invasion plasmid antigens that may be structural components of the Shigella invasion determinant; and (ii) invAKJH (mxi), which is necessary for insertion of invasion plasmid antigens into the outer membrane.

Construction and evaluation of live attenuated vaccine strains of Shigella flexneri and Shigella dysenteriae 1

Shigellosis is an invasive disease of the human colon which is particularly prevalent among children of the developing world. No proper vaccine is available to protect against this enteric disease. It is currently accepted that only live strains with attenuated virulence administered orally may elicit protective immunity at the level of the colonic mucosa, which is the exclusive site of multiplication of causative microorganisms such as Shigella flexneri and Shigella dysenteriae 1. We have constructed such vaccine candidates based on the destruction of virulence genes responsible for selected steps of the infection process. In S. flexneri, a combination of two mutations impairing cell-to-cell spread (icsA) and aerobactin production and transport (iuc, iut) which support growth within tissues provide a well tolerated and protective vaccine prototype against shigellosis in macaque monkeys. In S. dysenteriae 1, similar mutations are currently being introduced, in addition to one which eliminates the catalytic activity of Shiga toxin. These mutants and others will be tested soon in human phase I trials.

Construction of an auxotrophic Shigella flexneri strain for use as a live vaccine

A virulent Shigella flexneri serotype Y strain, SFL1, was made auxotrophic for aromatic metabolites, including p-aminobenzoic acid, which is not available in mammalian tissues, by transduction of a Tn10-inactivated aroD gene from Escherichia coli K-12 NK5131. One transductant, SFL114, selected for further studies, had the same biochemical and serological characteristics as the parent strain and the O-antigen patterns of the two strains were identical in SDS-PAGE and Western blot experiments. SFL114 was as invasive for cultured epithelial cells as SFL1, and both strains could escape from the phagocytic vacuole into the cytoplasm of the infected cells. However, the ability of SFL114 to multiply intracellularly was considerably reduced. When applied to the conjunctival sac of guinea pigs, the parent strain gave rise to keratoconjunctivitis, i.e. was Serény-positive, in 13 of 16 animals. By contrast, SFL114 was Serény-negative in all 11 guinea pigs tested. These in vitro and in vivo results suggest that the aromatic-dependent transductant S. flexneri SFL114 is attenuated and possesses properties desirable for a live vaccine.

Construction and evaluation of a double mutant of Shigella flexneri as a candidate for oral vaccination against shigellosis

Based on studies on the genetic and molecular basis of Shigella flexneri invasive properties, we have constructed and evaluated a double mutant of S. flexneri serotype 5 for utilization as a live attenuated oral vaccine against shigellosis. The first mutation, icsA, blocks intracellular spread of bacteria as well as cell-to-cell infection. It affects the capacity of the invasive pathogen to form large abscesses in epithelia. The second mutation, iuc, eliminates production of the siderophore aerobactin thus impairing growth of the bacterium within tissues. This double mutant, SC5700 appeared safe when administered intragastrically to macaque monkeys as three doses (5 x 10(10) c.f.u. each) at weekly intervals. Protection against a challenge by the wild type isolate (M90T) was observed 4 weeks after the last vaccine inoculation. Duration of carriage was considerably reduced as compared to the control group in which all animals had developed severe dysentery. Seroconversion against serotype 5 LPS as well as S. flexneri virulence associated polypeptides was also observed.

Loss of virulence in Shigella strains preserved in culture collections due to molecular alteration of the invasion plasmid

Fifty-two Shigella strains long preserved by three Japanese culture collections were examined for virulence. All of them were avirulent when judged by the focus-plaque assay and the ability to bind Congo red. Fifteen strains had a plasmid comparable in size to that responsible for epithelial invasiveness and were positive in hybridization tests with a probe derived from a plasmid cistron, virG. Twenty-four strains had a similar plasmid but were negative in hybridization tests. The remaining 13 strains were negative in all the five criteria for virulence. Similar studies made on one hundred Shigella strains isolated from 1967 to 1985 clearly demonstrated loss of virulence with prolonged time of storage.

Role of M cells in initial antigen uptake and in ulcer formation in the rabbit intestinal loop model of shigellosis

Strains of Shigella flexneri with different invasive and pathogenic potentials were inoculated into the intestinal lumen of acutely ligated loops in nonimmune rabbits. After 90 min, tissues processed for ultrastructural as well as light microscopy showed that the bacilli were phagocytosed by M cells over lymphoid follicles of Peyer's patches and carried in vacuoles into the epithelium. Nonpathogenic as well as pathogenic strains were readily taken up regardless of the presence of the 140-megadalton virulence plasmid. More virulent than avirulent shigellae were found in M cells at 90 min, reflecting replication or preferential uptake of the virulent strains. Heat-killed shigellae of the virulent strain were taken up by M cells to the same degree as the avirulent strains. Incubation of the bacteria for 18 h resulted in surface ulceration which was limited to epithelium overlying lymphoid follicles (M cell areas) in acute loops exposed to the virulent shigellae. Villus epithelium adjacent to the ulcerated follicular domes was intact, although there was mucus depletion. In the present study, we found that pathogenic shigellae appear to replicate in the M cells, escape from the phagocytic vesicles, and thereby initiate the ulcerations in this experimental model of dysentery. While initial antigen processing in the gut for a mucosal immune response may require uptake of luminal microorganisms by M cells, this may pose a threat under some circumstances.

Molecular cloning and characterization of chromosomal virulence region kcpA of Shigella flexneri

In Shigella flexneri, in addition to several well-recognized plasmid-borne virulence loci, at least three genetic loci implicated in pathogenesis have been recognized on the chromosome. To understand more about the pathogenesis of bacillary dysentery at a molecular level, the genetically recognized but previously unidentified KcpA region (one of the chromosomal regions near purE) was cloned and sequenced. A single translatable open reading frame encoding a 12310 Dalton protein corresponding to the minicell product was found. Immunofluorescence microscopy, as well as optical and electron microscopic comparison of tissue-cultured cells and guinea-pigs' eyes infected with wild-type or kcpA mutant bacteria, revealed that the kcpA product is required by invading bacteria for spread into adjacent cells.

Characterization of a cis regulatory DNA element necessary for formate induction of the formate dehydrogenase gene (fdhF) of Escherichia coli

The fdhF gene, encoding the selenopolypeptide of formate dehydrogenase (FDHH), has a -12/-24 nif-type consensus promoter. A cis-acting DNA element, which is required for the regulation of the promoter by formate under anaerobic conditions, has been identified. This regulatory sequence of about 25 bp in length is located 110 bp upstream of the transcription start site. By analysing a variety of mutant constructs in this region (5' deletions, internal deletions and point mutations) we were able to identify a hexanucleotide sequence -GTCACG-, which is important for the formate regulation of the fdhF promoter. The data also indicate that this element has many of the properties characteristic of eukaryotic enhancers.

Serological response to plasmid-encoded antigens in children and adults with shigellosis

The immunological response to plasmid-encoded antigens of virulent Shigella was determined in Thai children less than 4 yr of age and in Thai adults by immunoblot analysis and ELISA. Forty-two percent (8/19) of Thai children and 4% (1/22) of Thai adults with shigellosis developed a greater than or equal to 4-fold rise in IgG antibody titer to water-extracted antigens of Shigella flexneri M90T by ELISA (p = 0.006). Two children and one lactating mother with shigellosis developed a 4-fold rise in serum IgA antibody titers to water-extracted antigens of M90T. The results of the ELISA were confirmed by immunoblot analysis in all of the 41 paired sera examined. Five patients developed IgA, and four developed IgM, antibodies as detected by immunoblot analysis, that were not detected by ELISA. The reciprocal log2 geometric mean titers of antibodies to plasmid-encoded antigens in acute sera was higher in Thai adults than Thai children: IgG 7,265 versus 1,659; IgM 879 versus 480; and IgA 662 versus 60 (p less than 0.001). Thai adults had high titers of antibodies to plasmid-encoded antigens in their acute sera, but were susceptible to Shigella infections, although they were historically less susceptible than Thai children.

Nucleotide sequence of the invasion plasmid antigen B and C genes (ipaB and ipaC) of Shigella flexneri

The nucleotide sequence of a 4.8 kilobase (kb) HindIII fragment from pWR100, the virulence plasmid of Shigella flexneri 5, was determined and analysed. This fragment encodes polypeptides b (62 kilodalton, kD) and c (43 kD) which have already been described as two of the four immunogenic polypeptides of Shigellae. The nucleotide sequence revealed that in addition to the ipaB and ipaC genes encoding polypeptides b and c, a third complete open reading frame was found within the fragment. The gene, named ippI, encoded a 17 kD polypeptide. The deduced amino acids sequence of polypeptides b and c showed no signal peptide but presence of highly hydrophobic domains compatible with a transmembraneous location. The surprising A and T richness of the three genes as compared with the Escherichia coli and Shigella genomes, resulted in a biased codon usage, and raises the question of the origin of the sequences.

A basic replicon of virulence-associated plasmids of Shigella spp. and enteroinvasive Escherichia coli is homologous with a basic replicon in plasmids of IncF groups

Shigella species and enteroinvasive Escherichia coli strains carry a large (120- to 140-megadalton) plasmid called pINV, which contains genes essential for the invasiveness of these pathogens. Hybridization with specific probes derived from the RepFIC and RepFIB replicons of the IncF1 Ent plasmid P307 showed that pINVs present in 35 clinical isolates are homologous with RepFIC but not RepFIB, regardless of the serogroup of the Shigella or E. coli strain. RepFIC of P307, in turn, is very similar to RepFIIA replicons of IncFII R plasmids. These and other related replicons constitute the RepFIIA family. With one pINV, pWR110, a plasmid of Shigella flexneri 5, we demonstrated the existence of a functional replicon, RepINV, with a restriction map similar to that of RepFIIA of plasmid R1. We isolated the putative inc RNA coding region of RepINV, which is a major determinant of incompatibility. The nucleotide sequence of the RepINV-inc RNA-coding region was determined and compared with the corresponding sequences of RepFIC and RepFIIA. The differences were small, but apparently were sufficient to affect the target specificity of the inc RNAs, thus rendering the replicons compatible with each other. We conclude that pINVs present in Shigella spp. and enteroinvasive E. coli constitute a homogeneous group, containing one basic replicon that belongs to the RepFIIA family of replicons.

Combined parenteral and oral immunization results in an enhanced mucosal immunoglobulin A response to Shigella flexneri

Achieving a vigorous secretory immunoglobulin A (IgA) response in intestinal secretions usually requires multiple doses of antigen given orally, while systemic immunity is more easily attained by parenteral immunization. This study examines the role of combined parenteral and oral immunizations to enhance the early mucosal immune response to an enteropathogen. We have used a chronically isolated intestinal-loop model in rabbits as a probe to monitor kinetically the initial (primary) local immune response to shigella lipopolysaccharide (LPS) following combinations of parenteral immunization intramuscularly (i.m.) and oral stimulation with shigellae. Predictably, effective stimulation of systemic immunity was elicited when heat-killed preparations of Shigella sp. strain X16 were given i.m., as shown by strong serum IgG and weak intestinal IgA activity to shigella LPS. A single oral dose of live Shigella sp. strain X16 given to unprimed rabbits elicited only a typical weak IgA response in intestinal secretions. However, when an i.m. dose of heat-killed shigellae was followed 1 day later by an oral dose of live Shigella sp. strain X16, a hyperstimulation of the early secretory IgA response was elicited, and the response reached levels found previously only after multiple oral administrations of live shigellae. This stimulation did not require the use of an adjuvant. At the same time, the animals receiving this combined oral and i.m. regimen had a lower IgG antishigella LPS activity in serum compared with their response after receiving parenteral antigen in adjuvant alone. These findings indicate that while a dichotomy exists between the systemic and mucosal immune responses, careful orchestration of the stimulatory events can promote a vigorous early local IgA response.

Development of an auxotrophic oral live Shigella flexneri vaccine

An oral live attenuated Shigella flexneri vaccine candidate strain was constructed by making it auxotrophic and dependent on aromatic metabolites not available in mammalian tissues. An aroD gene of Escherichia coli K12 strain NK 5131, inactivated by insertion in it of the Tn 10 transposon, was transduced using phage P1 into a virulent S. flexneri serotype Y strain (Sfl 1) isolated from a patient with bacillary dysentery. One of the transductant strains Sfl 114 was found to invade HeLa cells in vitro, to cause plaque formation in HeLa monolayers (i.e. maintain intracellular multiplication in vitro), but to be unable to cause keratoconjunctivitis in guinea-pig eyes. When the strain was fed to Macacca fascicularis monkeys it was well tolerated, excreted for 1-4 days, and found to elicit a local intestinal sIgA and serum IgA, IgM and IgG responses. Monkeys challenged with 100 ID50 dose (1 X 10(11) bacteria) of the virulent parent Sfl 1 strain were completely protected from development of diarrhoea. Coloscopy of the monkeys and the sampling of intestinal biopsies showed that the vaccine protected against the surface epithelial erosions and ulcerations seen in unimmunized monkeys. Killing of invading virulent shigellae apparently took place intracellularly in the mucosa suggesting that cellular immune mechanisms played a role in the elicited host defence. The constructed S. flexneri Sfl 114 strain has the properties of a promising shigella vaccine and will next be the subject of studies with human volunteers.

Analysis and genetic manipulation of Shigella virulence determinants for vaccine development

Shigellosis is a major public health problem in developing countries. Current epidemics of Shigella dysenteriae serotype 1 strains are particularly serious and are characterized by high mortality rates. A high proportion of the isolates are resistant to many of the antibiotics currently in use in these countries, a feature which seriously compromises clinical treatment of the infections. Efficacious vaccines are thus urgently needed. Basic studies on Shigella virulence factors, infections in laboratory models, and host responses has led to the development of several strategies for the production of vaccines. All of these are live oral vaccines involving bacteria capable of at least limited survival in the animal intestine and of carrying selected antigens to the mucosal immune system. One type of vaccine involves non-pathogenic shigellae, attenuated either by introduction of a requirement for aromatic amino acids (aroD) or by loss of the large plasmid that specifies bacterial invasion of the mucosal epithelium. S. dysenteriae 1 strains under development as vaccines need to be engineered to eliminate high level Shiga toxin production, and a rapid and effective method to achieve this was recently elaborated. The second type of vaccine is represented by hybrid strains consisting of a carrier organism, such as an attenuated Salmonella or an Escherichia coli K-12 strain carrying the Shigella invasion plasmid, and the selected foreign antigen that it produces, in all cases so far the Shigella O antigen polysaccharide.

Comparison of the ability of enteroinvasive Escherichia coli, Salmonella typhimurium, Yersinia pseudotuberculosis, and Yersinia enterocolitica to enter and replicate within HEp-2 cells

Salmonella typhimurium, enteroinvasive Escherichia coli, Yersinia pseudotuberculosis, and Yersinia enterocolitica possess the ability to enter intestinal epithelial cells. We used a quantitative tissue culture model employing HEp-2 cells to compare the abilities of these bacteria to enter epithelial cells. S. typhimurium and Yersinia species were highly infective for HEp-2 cells but were unable to replicate extensively intracellularly. Enteroinvasive E. coli exhibited low infectivity but replicated extensively intracellularly. The growth of enteroinvasive E. coli led to destruction of the HEp-2 monolayer, whereas Yersinia spp. and S. typhimurium were maintained intracellularly for prolonged periods without damage to the monolayer. The ability of enteroinvasive E. coli to enter HEp-2 cells required prior growth at 37 degrees C; neither S. typhimurium nor Yersinia spp. exhibited this temperature dependence for cell entry. An E. coli K-12 derivative containing a 230-kilobase plasmid from enteroinvasive E. coli was constructed. This derivative shared all the invasive characteristics of the parental enteroinvasive strain, suggesting that determinants required for cell entry and intracellular multiplication were at least partially plasmid encoded. An HB101 derivative containing a cloned invasion determinant from Y. pseudotuberculosis was constructed in our laboratory. HEp-2 monolayers were coinfected with these two K-12 derivatives to compare invasion determinants from enteroinvasive E. coli with those of Y. pseudotuberculosis in a common genetic background. Results from these experiments suggest that these organisms reside within separate intracellular compartments.

Characterization of Shigella flexneri sequences encoding congo red binding (crb): conservation of multiple crb sequences and role of IS1 in loss of the Crb+ phenotype

The ability to bind Congo red (Crb+) is associated with virulence of Shigella flexneri and is encoded by a large, 220-kilobase plasmid. We cloned fragments of this plasmid to isolate the sequences encoding Congo red binding, to determine the degree of conservation of these sequences among S. flexneri strains, and to study the molecular basis for loss of the Crb+ phenotype. At least two separate BamHI fragments cloned into plasmid vectors encode Congo red binding in E. coli or S. flexneri. One Crb+ clone, pTKS2, contains a copy of IS1 adjacent to the crb sequences. IS1 appears to be responsible for deletions leading to loss of Congo red binding in this clone. In addition, this clone was found to integrate into the chromosome at relatively high frequency. Integration resulted in loss of the Crb+ phenotype. A second clone, pTKS15, which has only limited homology to pTKS2, also encodes Congo red binding. The Crb+ phenotype of transformants carrying pTKS15 was detected at 37 degrees C but not at 30 degrees C, and thus it resembles Congo red binding in wild-type S. flexneri. HindIII digests of plasmid DNA from 10 different S. flexneri strains were hybridized to both of these Crb+ clones and to an IS1 probe. More than one fragment hybridized to pTKS2 or pTKS15. In general, the sizes of these fragments were the same in S. flexneri strains of different serotypes, indicating conservation of these sequences. Three of five copies of IS1 were also found on the large S. flexneri plasmids. Two of the copies were on fragments of the same size in each strain. Analysis of Crb- derivatives of the 10 strains indicated that, although IS1 may be closely linked to crb sequences on the 220-kilobase plasmid, it is not responsible for the majority of deletions of this plasmid associated with loss of Congo red binding.

Serum immune response to Shigella protein antigens in rhesus monkeys and humans infected with Shigella spp

The serum antibody response to proteins encoded by the virulence-associated plasmid of Shigella flexneri was determined in monkeys challenged with virulent S. flexneri serotype 2a. With water-extractable antigen in an enzyme-linked immunosorbent assay, a significant increase in antibody titer against proteins from a plasmid-carrying, virulent strain of S. flexneri serotype 5 could be demonstrated in convalescent sera. There were minimal antibody titers against proteins from an avirulent (plasmid-free) organism. Previously identified plasmid-coded polypeptides a, b, c, and d were predominant antigens recognized by a majority of the convalescent sera in immunoblots. An additional 140-megadalton plasmid-coded polypeptide was also recognized by half of the sera. Convalescent serum from an infected monkey recognized antigens on the bacterial surface in several different plasmid-containing Shigella species and in an enteroinvasive Escherichia coli strain. A survey of sera obtained from children 5 to 10 years of age who had been infected with S. flexneri or S. sonnei revealed high enzyme-linked immunosorbent assay titers in both acute and convalescent sera against a water extract from a virulent Shigella strain. In contrast, children under 3 years of age had no antibody titer in either acute or convalescent sera against the virulence-associated shigella proteins, while 3- to 4-year-old children mounted an immune response against these proteins only in convalescence.

Plaque formation by virulent Shigella flexneri

An in vitro tissue culture plaque assay was developed to investigate the intracellular replication and intercellular spread of virulent shigellae. Shigella plaques were formed in HeLa cell monolayers in the presence of an agarose overlay containing tissue culture medium and gentamicin, which eliminated extracellular bacterial growth. Microscopically, the plaques were characterized by a central area of dead host cells surrounded by cells infected with shigellae. Cells further away from the plaque center were uninfected. Inclusion of chloramphenicol or nalidixic acid in the overlay completely abolished plaque formation. Plaque formation was completely inhibited when infected monolayers were shifted from 37 to 30 degrees C. Shifting infected monolayers from 30 degrees C, where plaques do not form, to 37 degrees C resulted in the formation of plaques. Cultures of Shigella boydii, Shigella sonnei (form I), and all six serotypes of Shigella flexneri produced plaques. Shigellae isolated from plaques were Sereny test positive, contained a 140-megadalton plasmid, and were gentamicin sensitive. Noninvasive shigellae did not form plaques.

Modulation of in vitro natural cell-mediated activity against enteropathogenic bacteria by simple sugars

Lymphoid cells from mouse Peyer's patches and spleens were tested in a 2-h in vitro assay for their natural activity against the enteropathogenic bacteria Salmonella typhimurium, Salmonella enteritidis, Salmonella tel aviv, and Shigella sp. X16. The antibacterial activity expressed by normal cells was detected against all the bacterial strains tested with the exception of Peyer's patch lymphocytes against S. tel aviv and splenocytes against Shigella sp. X16. To determine whether the different expression of natural antibacterial activity might be due to lectin-like proteins interacting with the saccharidic moieties of the bacterial wall, 11 simple sugars were preincubated with the effector cells before the in vitro assays. We found that some of them could block the natural antibacterial activity as well as induce antibacterial activity when this was not spontaneously expressed. Interestingly, a different panel of sugars among those employed was observed to affect the antibacterial activities for each of the above-mentioned bacterial targets and each effector cell. However, the same panel of sugars was able to block or stimulate the lymphocyte activity when bacteria with the same somatic antigens as two substrains of S. typhimurium and one strain of Salmonella schottmuelleri were employed. To further investigate the interaction between effector cells and bacteria, effector cells or Shigella sp. X16 targets were treated with proteolytic, glycolytic, and lipolytic enzymes before the in vitro assays. Furthermore, EDTA was used to analyze the role of divalent cations in this experimental system. The results obtained suggest that lectin-like proteins playing a role in this interaction are present not only on lymphocytes but also on bacteria and that divalent cations are essential for the expression of in vitro antibacterial activity.

Effect of antigen form on local immunoglobulin A memory response of intestinal secretions to Shigella flexneri

An enhanced memory response, as shown by increased titers of specific immunoglobulin A (IgA), was seen in intestinal secretions from isolated Thiry-Vella loops in rabbits primed orally with live, locally invasive Shigella sp. X16 and challenged 60 days later with a single oral dose of the same antigen. Heat-killed shigella preparations, when used as either the priming or challenge antigen, did not elicit such a memory response in this system. In the present study, the role of antigen form and dosage in eliciting the enhanced local IgA response was investigated. A noninvasive strain, Shigella flexneri 2457-0, was capable of significantly enhancing the mucosal IgA memory response, whereas heat-killed Shigella sp. X16 was unable to augment the local IgA response, even when the priming dose was increased 100-fold. A proposed mucosal adjuvant, DEAE-dextran, given orally with live Shigella sp. X16, did not enhance the local IgA response. Viable, noninvasive shigellae were effective priming agents in enhancing the local IgA memory response. The poor mucosal response to heat-killed shigella preparations is thought to be related to an ineffective delivery of nonviable bacterial antigens into gut-associated lymphoid tissues. The ability of the live, noninvasive strain to elicit a vigorous local IgA memory response when given orally to rabbits was consistent with previous findings that live preparations elicit the best mucosal IgA response.

Small virulence plasmid of Shigella dysenteriae 1 strain W30864 encodes a 41,000-dalton protein involved in formation of specific lipopolysaccharide side chains of serotype 1 isolates

A 6-megadalton plasmid, pHW400, of Shigella dysenteriae 1 strain W30864 was previously found to specify one or more functions for O-antigen production and bacterial virulence (H. Watanabe and K. N. Timmis, Infect. Immun. 43:391-396, 1984). The region of pHW401, a Tn801-tagged derivative of pHW400, responsible for O-antigen production has been localized by gene cloning and Tn5 transposon mutagenesis. Analysis of lipopolysaccharide isolated from S. dysenteriae 1 bacteria carrying mutant plasmids revealed that the determinant for O-antigen synthesis, designated rfp, codes for a function involved in the formation of the O-polysaccharide side chain structure of lipopolysaccharide. Analysis of radioactively labeled proteins synthesized in minicells of Escherichia coli carrying mutant plasmids identified the product of the rfp gene as a 41,000-dalton protein. Southern hybridization with a DNA fragment carrying the rfp gene demonstrated that this determinant is present on 6-megadalton plasmids in other isolates of S. dysenteriae 1 but is not present at all in a variety of other Shigella, E. coli, and Salmonella typhimurium strains that were tested.

Bacteriophage Mu d1(Apr lac) generates vir-lac operon fusions in Shigella flexneri 2a

Previous studies have demonstrated that expression of virulence in Shigella spp. is controlled by growth temperature. To study the regulation of virulence (vir) genes, we set out to develop a rapid, easily-assayed phenotype with which to measure expression of virulence. This report described a procedure for isolating vir-lac operon fusions in S. flexneri 2a by using the specialized transducing bacteriophage Mu d1(Apr lac) of Casadaban and Cohen (M. Casadaban and S. N. Cohen, Proc. Natl. Acad. Sci. U.S.A. 76:4530-4533, 1976). Mu d1(Apr lac) lysogens were isolated and screened for loss of virulence and for temperature-dependent expression of the lactose genes on Mu d1(Apr lac). A recombinant plasmid carrying the Mu immunity gene was also introduced into lysogens of interest to stabilize the Mu d1(Apr lac) insertion and prevent possible thermal induction at 37 degrees C. The mutant which we isolated failed to penetrate tissue culture cells in the assay for virulence and produced almost 15-fold more beta-galactosidase when grown at 37 degrees C than when grown at 30 degrees C. The site of insertion of Mu d1(Apr lac) in this strain was shown to be in the 140-megadalton plasmid pSf2a140, which is known to be associated with virulence. P1L4-mediated transduction of the insertion into a virulent recipient demonstrated genetic linkage of Mu d1(Apr lac) with loss of virulence and temperature-dependent expression of beta-galactosidase. All of these features fulfill the phenotype expected for a Mu d1(Apr lac)-induced vir-lac operon fusion. This mutant provides us with a means of measuring expression of a gene function required for virulence by assaying for beta-galactosidase. The insertion will also serve as a starting point for mapping of genes on pSf2a140 which are necessary for expression of virulence.

A small plasmid in Shigella dysenteriae 1 specifies one or more functions essential for O antigen production and bacterial virulence

The role of plasmids in the virulence of Shigella dysenteriae 1 W30864, which contains at least five species, was investigated. By means of a standard plasmid-curing procedure, that is, bacterial cultivation at an elevated temperature, five virulence-deficient derivatives were obtained. One of these lacked a small, 6-megadalton plasmid, designated pHW400, exhibited reduced invasiveness for HeLa cells, and failed to produce the somatic antigen. Transposon tagging of the pHW400 plasmid to produce pHW401 and the transfer of this derivative into a variant of strain W30864 lacking pHW400 confirmed the conclusion that the pHW400 plasmid encodes one or more functions involved in O antigen (lipopolysaccharide) biosynthesis and bacterial virulence. A plasmid of similar size was detected in all of the three other strains of S. dysenteriae 1 examined.

New knowledge on pathogenesis of bacterial enteric infections as applied to vaccine development

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Antibody-dependent cell-mediated antibacterial activity of intestinal lymphocytes with secretory IgA

Secretory antibodies of the IgA class (sIgA) are thought to have an important role in the defence against bacteria at mucosal surfaces--the level at which the infectious agents first come into contact with the host. However, the mechanism by which sIgA exert their antibacterial activity is still a matter of debate. After the recent discovery of receptors for the Fc portion of IgA (RFc alpha) on lymphocytes, monocytes and granulocytes of human, rabbit, guinea pig and mouse origin, it has been hypothesized that IgA also mediate antibody-dependent cellular cytotoxicity (ADCC). Indeed, ADCC mediated by human leukocytes against bacteria has been demonstrated in the presence of human circulating IgA. As RFc alpha have also been shown to bind sIgA, we decided to investigate whether sIgA could mediate antibacterial ADCC when bound to lymphocytes from the murine gut-associated lymphoid tissues (GALT) which first interact with the invading bacteria. By using Shigella X16 (a hybrid strain between the enteric pathogen Shigella flexneri and Escherichia coli) as target in an in vitro assay that measures cell-mediated antibacterial responses, we found that murine lymphocytes from GALT but not from other tissues are able to exert natural antibacterial activity against Shigella X16, and that sIgA significantly and specifically increase the natural antibacterial activity of GALT lymphocytes from mice and induce antibacterial activity in cells from the spleen, but not from the thymus or popliteal lymph nodes. Thus, we now propose a new role for sIgA in protecting the host against infectious agents at the mucosal level.

Characterization of virulence plasmids and plasmid-associated outer membrane proteins in Shigella flexneri, Shigella sonnei, and Escherichia coli

The 140-megadalton plasmids of Shigella flexneri serotypes 1, 3, and 5, in addition to the 120-megadalton plasmid of Shigella sonnei, are associated with virulence. The present study showed that a 140-megadalton plasmid is also associated with virulence in Escherichia coli. When these plasmids were cleaved with EcoRI or BamHI restriction endonucleases, considerable homology was evident in plasmids from S. sonnei strains, whereas only a few common fragments were observed among the S. flexneri and enteroinvasive E. coli plasmids. Nitrocellulose filter hybridization demonstrated that, despite variations in restriction sites, all these plasmids shared a considerable complement of homologous sequences. Minicell-producing strains were obtained by N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis. Transmission electron microscopy of infected HeLa cells showed that minicells from invasive strains retained the invasive phenotype. Sixteen polypeptides were labeled when S. flexneri 5 minicells were incubated with [35S]methionine. Fourteen of these plasmid-coded polypeptides were associated with the outer membrane in invasive strains of S. flexneri 5, and nine polypeptides of similar molecular weight were labeled in the outer membrane of invasive strains of S. flexneri 3, S. sonnei, and E. coli. Seven of the S. flexneri 5 polypeptides were not labeled in a noninvasive strain which had sustained a large deletion in the virulence-associated plasmid, and none were labeled in minicells which no longer harbored this plasmid.

Alterations in the pathogenicity of Escherichia coli K-12 after transfer of plasmid and chromosomal genes from Shigella flexneri

A 140-megadalton plasmid (pWR110), which has previously been associated with virulence in Shigella flexneri, was transferred to Escherichia coli K-12. Segments of S. flexneri chromosomal material were then transferred to the plamid-bearing K-12 strains. The virulence of these transconjugant hybrids was assessed in the HeLa cell model, in ligated rabbit ileal loops, or in the Sereny test. A K-12 strain which harbored only pWR110 invaded HeLa cells, produced minimal lesions in the rabbit ileal mucosa, and was negative in the Sereny test. Plasmid-containing K-12 hybrids which had incorporated various shigella chromosomal regions gave differential reactions in the rabbit ileal loops and in the Sereny test. Analysis of these transconjugants indicated that three regions were linked with virulent phenotypes. These included the his region (when the genes responsible for O-antigen synthesis were cotransferred) and the kcp locus (linked to the lac-gal region). Either of these chromosomal regions was sufficient to allow invasion of the rabbit ileal mucosa. In addition to both of these regions, another shigella chromosomal segment linked to the arg and mtl loci was necessary for fluid production in the rabbit ileal loop and for a positive Sereny reaction. Thus, derivatives of an E. coli K-12 strain, constructed by the stepwise conjugal transfer of a large plasmid and three chromosomal segments from S. flexneri, appeared to contain the necessary determinants for full pathogenicity in a variety of laboratory models.

HeLa cell invasiveness and O antigen of Shigella flexneri as separate and prerequisite attributes of virulence to evoke keratoconjunctivitis in guinea pigs

Many rough mutants selected from isogenic smooth virulent and avirulent strains of Shigella flexneri were examined for virulence, using tissue culture infection and Sereny tests. Many of the rough mutants isolated from a virulent smooth strain were capable of penetrating tissue culture cells but incapable of producing a positive Sereny test. In contrast, we could not obtain from smooth avirulent strains any rough mutants capable of penetrating HeLa cells. Chemical analysis of lipopolysaccharide of some representative rough strains showed several patterns of sugar composition with a range of from Ra to Re chemotypes. There was no correlation between HeLa cell invasiveness and chemotypes of lipopolysaccharides, thus indicating little significance of oligosaccharides of the rough core as well as O antigens in the ability of S. flexneri to penetrate HeLa cells. When these invasive rough strains were given O antigen genes from a smooth avirulent Shigella Hfr strain, most of the transconjugants that expressed O antigens regained the ability to evoke keratoconjunctivitis in guinea pigs. We also examined the chromosomal loci of HeLa cell invasion by transferring carbohydrate fermentation genes of Escherichia coli K-12 Hfr and found two chromosomal loci, the rha and lac-gal regions, which control the ability to penetrate HeLa cells. These results suggested that O antigens and ability to penetrate tissue culture cells are independent and prerequisite attributes of virulence in Shigella flexneri to evoke keratoconjunctivitis in guinea pigs.

Involvement of a plasmid in the invasive ability of Shigella flexneri

Representative Shigella flexneri strains were studied to determine whether plasmids are involved in their virulence. All invasive S. flexneri strains, irrespective of serotype, were found to harbor a large plasmid of approximately 140 megadaltons in size, although some strains carried additional plasmid species. Spontaneous variants of strains of serotypes 1, 2, and 5 had lost this 140-megadalton plasmid and had concomitantly become avirulent, i.e., could neither invade HeLa cell monolayers nor produced keratoconjunctivitis in guinea pigs. To monitor plasmid transfer, the 140-megadalton plasmid of strain M90T (serotype 5) was tagged with the kanamycin resistance transposon Tn5. This tagged plasmid, pWR110, was not self-transmissible, but was mobilized by one of several different conjugative plasmids into avirulent derivatives of the heterologous serotypes 1 and 2 which had lost the comparable large plasmid. Transconjugants of both serotypes which had received pWR110 regained virulence. These data directly demonstrate that this large S. flexneri plasmid encodes or regulates some function(s) required for epithelial cell penetration.

Shigella sonnei plasmids: evidence that a large plasmid is necessary for virulence

Virulent form I Shigella sonnei strains contain a 120-megadalton plasmid that is absent in their form II derivatives, which are always avirulent and devoid of O side chains. In the present study, 165 biochemical and antibiotic traits were assessed, but no experimentally useful phenotype could be associated with this large form I plasmid. Therefore, the form I plasmids of several S. sonnei strains were tagged with the antibiotic resistance transposons Tn3, Tn5, or Tn10. Transposon-tagged form I plasmids were not self-transmissible, but could be mobilized by the plasmid R386. Form II S. sonnei transconjugants for the form I plasmid acquired both virulence and the ability to synthesize form I antigen, establishing that these properties are plasmid mediated. Further studies indicate that this 120-megadalton form I plasmid is physically unstable in any of several host bacteria and suggest that it is a member of the FI incompatibility group. Also, two commonly observed, small plasmids of S. sonnei, of 3.2 and 3.9 megadaltons, were shown to encode either colicin E1 production or resistance to streptomycin and sulfonamide, respectively.

HeLa cell infection by Yersinia enterocolitica: evidence for lack of intracellular multiplication and development of a new procedure for quantitative expression of infectivity

The in vitro invasive properties of bacteria have frequently been studied by the use of HeLa cell cultures in chamber slides, using microscopic examination to enumerate intracellular bacteria. When this system was used to examine invasive properties of Yersinia enterocolitica, it resulted in rapid internalization of high numbers of bacteria during the infection phase which prevented subsequent discrimination of intracellular multiplication. A modified procedure was developed which standardized the ratio of bacteria to HeLa cells (i.e., multiplicity), the time for the infection phase, and the addition of specific antiserum with gentamicin for restricting bacterial uptake during the intracellular growth phase. Studies with this modified chamber slide system found that strains of human isolates of Y. enterocolitica (serotypes O:3, O:8, O:5,27, and O:6,30) exhibited different degrees of cell infection but did not multiply intracellularly. A second test system was developed that used roller tubes and viable cell counts for the enumeration of intracellular bacteria. This roller tube system confirmed that internalized bacteria did not multiply inside HeLa cells over a 24-h period. The roller tube system with viable cell counts for enumeration is a simplified technique for quantitative comparison of in vitro infectivity of HeLa cells by Y. enterocolitica.

Role of antigen form in development of mucosal immunoglobin A response to Shigella flexneri Antigens

One major stumbling block in the development of an effective means to immunize against shigellosis and other enteric diseases has been the lack of a means to assess sequential mucosal immune responses to different potential immunogens. In the present study, we compared the abilities of live invasive organisms, noninvasive organisms, and nonviable antigen preparations of shigella to elicit mucosal immune responses. Whereas previous studies have found that effective immunity was produced best by vaccination with live invasive strains of shigella, in the present study, live noninvasive strains that did not produce any histopathological damage were consistently able to produce local (immunoglobulin A) immune responses as vigorous as those of the invasive strains. Further, acetone-killed shigella antigen was also an effective mucosal immunogen, whereas hot phenol-water-extracted shigella lipopolysaccharide was ineffective, possibly due to the method of preparation. A single oral or parenteral priming was ineffective in enhancing the mucosal immune response when restimulated 1 month later with the same antigen. However, a mucosal memory response was found to be present several months after a triple mucosal stimulation with a locally invasive vaccine strain.

Inhibition of clinically significant bacterial organisms in vitro by 2-acetylpyridine thiosemicarbazones

Antibacterial activity of 65 2-acetylpyridine thiosemicarbazones and related compounds was determined by using clinical isolates of nine bacterial genera. Minimal inhibitory concentrations (MICs) of 0.002 to 0.062 micrograms/ml were obtained with 23% of the compounds for Neisseria gonorrhoeae and 0.016 to 0.062 micrograms/ml with 17% of the compounds for N. meningitidis. Staphylococcus aureus was inhibited in the MIC range of 0.125 to 0.5 micrograms/ml by 18% of the thiosemicarbazones, whereas 26% inhibited group D enterococcus with an MIC of 0.25 to 2.0 micrograms/ml. Poor antibacterial activity was shown toward the gram-negative bacilli, i.e., Pseudomonas, Klebsiella-Enterobacter, Shigella, Escherichia coli, and Proteus.