Association of viable and inactivated Salmonella typhimurium 395 MS and MR 10 with HeLa cells

Salmonella Intracellular Lifestyles and Their Impact on Host-to-Host Transmission

More than a century ago, infections by Salmonella were already associated with foodborne enteric diseases with high morbidity in humans and cattle. Intestinal inflammation and diarrhea are hallmarks of infections caused by nontyphoidal Salmonella serovars, and these pathologies facilitate pathogen transmission to the environment. In those early times, physicians and microbiologists also realized that typhoid and paratyphoid fever caused by some Salmonella serovars could be transmitted by "carriers," individuals outwardly healthy or at most suffering from some minor chronic complaint. In his pioneering study of the nontyphoidal serovar Typhimurium in 1967, Takeuchi published the first images of intracellular bacteria enclosed by membrane-bound vacuoles in the initial stages of the intestinal epithelium penetration. These compartments, called Salmonella-containing vacuoles, are highly dynamic phagosomes with differing biogenesis depending on the host cell type. Single-cell studies involving real-time imaging and gene expression profiling, together with new approaches based on genetic reporters sensitive to growth rate, have uncovered unprecedented heterogeneous responses in intracellular bacteria. Subpopulations of intracellular bacteria displaying fast, reduced, or no growth, as well as cytosolic and intravacuolar bacteria, have been reported in both in vitro and in vivo infection models. Recent investigations, most of them focused on the serovar Typhimurium, point to the selection of persisting bacteria inside macrophages or following an autophagy attack in fibroblasts. Here, we discuss these heterogeneous intracellular lifestyles and speculate on how these disparate behaviors may impact host-to-host transmissibility of Salmonella serovars.

Interactions of Salmonella with animals and plants

Salmonella enterica species are Gram-negative bacteria, which are responsible for a wide range of food- and water-borne diseases in both humans and animals, thereby posing a major threat to public health. Recently, there has been an increasing number of reports, linking Salmonella contaminated raw vegetables and fruits with food poisoning. Many studies have shown that an essential feature of the pathogenicity of Salmonella is its capacity to cross a number of barriers requiring invasion of a large variety of cells and that the extent of internalization may be influenced by numerous factors. However, it is poorly understood how Salmonella successfully infects hosts as diversified as animals or plants. The aim of this review is to describe the different stages required for Salmonella interaction with its hosts: (i) attachment to host surfaces; (ii) entry processes; (iii) multiplication; (iv) suppression of host defense mechanisms; and to point out similarities and differences between animal and plant infections.

Transposon mutagenesis of Salmonella enterica serovar Enteritidis identifies genes that contribute to invasiveness in human and chicken cells and survival in egg albumen

Salmonella enterica serovar Enteritidis is an important food-borne pathogen, and chickens are a primary reservoir of human infection. While most knowledge about Salmonella pathogenesis is based on research conducted on Salmonella enterica serovar Typhimurium, S. Enteritidis is known to have pathobiology specific to chickens that impacts epidemiology in humans. Therefore, more information is needed about S. Enteritidis pathobiology in comparison to that of S. Typhimurium. We used transposon mutagenesis to identify S. Enteritidis virulence genes by assay of invasiveness in human intestinal epithelial (Caco-2) cells and chicken liver (LMH) cells and survival within chicken (HD-11) macrophages as a surrogate marker for virulence. A total of 4,330 transposon insertion mutants of an invasive G1 Nal(r) strain were screened using Caco-2 cells. This led to the identification of attenuating mutations in a total of 33 different loci, many of which include genes previously known to contribute to enteric infection (e.g., Salmonella pathogenicity island 1 [SPI-1], SPI-4, SPI-5, CS54, fliH, fljB, csgB, spvR, and rfbMN) in S. Enteritidis and other Salmonella serovars. Several genes or genomic islands that have not been reported previously (e.g., SPI-14, ksgA, SEN0034, SEN2278, and SEN3503) or that are absent in S. Typhimurium or in most other Salmonella serovars (e.g., pegD, SEN1152, SEN1393, and SEN1966) were also identified. Most mutants with reduced Caco-2 cell invasiveness also showed significantly reduced invasiveness in chicken liver cells and impaired survival in chicken macrophages and in egg albumen. Consequently, these genes may play an important role during infection of the chicken host and also contribute to successful egg contamination by S. Enteritidis.

Composition and formation of flame cell caps: A substratum for the attachment of micro-organisms to sea horse epidermis

The epidermis of the sea horseHippocampus kudais characterised by flame cone cells, each of which protrudes 20-40 μm above the surface and is covered by a prominent mucous cap. Unlike normal surface cells, the mucoid caps can support epiphytic microbial growth.

Histochemically the mucous cap is a neutral mucopolysaccharide-protein complex possessing 1,2 glycol groups and SH-groups; acid mucopolysaccharides are absent. The acid mucopolysaccharide glycocalyx of unmodified surface cells is absent from the mucous cap surface.

Ultrastructurally two types of vesicle can be distinguished in flame cells. Type I is oval (0·3 × 0·6 um) with contents of medium electron density and occurs principally in mature flame cells. Type II, seen only in developing cells, is spherical (0·4 μm) and contains rod-like subunits characteristic of the cap mucous. Secretion is mediated by channels formed by smooth endoplasmic reticulum. Unmodified surface cells also secrete their vesicular contents, which resemble the Type I vesicles of flame cells and are similar to the goblet mucous vesicles of other fish, through similar channels.

Flame cell mucous caps, by virtue of their histochemistry, possible provide a suitable substratum for the adhesion and growth of epiphytes which in turn may afford protection against cnidarian nematocysts.

Immunological responses induced by asd and wzy/asd mutant strains of Salmonella enterica serovar Typhimurium in BALB/c mice

Attenuated bacteria have long been developed as vaccine candidates but can have some disadvantages, such as the potential for damage to immune organs due to insufficient clearance. To minimize these disadvantages, we generated Salmonella enterica serovar Typhimurium mutants SHJ2104 (asd::cm) and HTSaYA (wzy::km, asd::cm). The wzy gene codes for the O-antigen polymerase, which is involved in lipopolysaccharide (LPS) biosynthesis, and asd codes for aspartate beta-semialdehyde dehydrogenase, which participates in cell wall formation. The strains synthesized LPS with a short-chain length, and showed lower cytotoxicity and reduced intracellular proliferation in animal cells compared to wild-type bacteria. After oral infection, the mutants were cleared in immune tissues, including the Peyer's patch, mesenteric lymph node, and spleen, within 5 days. The LD50 of the mutants in Balb/c mice was estimated to be 10(6) higher than wild-type bacteria when administered either via an oral or i.p. route, indicating that the two strains are highly attenuated. To compare the immune response to and protective effects of the mutants against wild-type bacterial infection, we inoculated the mutants into mice via an oral (1x10(10)CFU) or i.p. (1x10(7) CFU) route once or twice at a two week interval. All immune responses, such as serum IgG and secretory IgA levels, cytokine production, and delayed hypersensitivity, were highly induced by two rounds of immunization. HTSaYA and SHJ2104 induced similar immune responses, and mice immunized with HTSaYA or SHJ2104 via an i.p. route were protected against wild-type Salmonella infection even at 100-fold of the LD(50) (5x10(6) CFU). Taken together, these data indicate that HTSaYA and SHJ2104 could be developed as live attenuated Salmonella vaccine candidates.

O-antigen-negative Salmonella enterica serovar Typhimurium is attenuated in intestinal colonization but elicits colitis in streptomycin-treated mice

Lipopolysaccharide (LPS) is a major constituent of the outer membrane and an important virulence factor of Salmonella enterica subspecies 1 serovar Typhimurium (serovar Typhimurium). To evaluate the role of LPS in eliciting intestinal inflammation in streptomycin-treated mice, we constructed an O-antigen-deficient serovar Typhimurium strain through deletion of the wbaP gene. The resulting strain was highly susceptible to human complement activity and the antimicrobial peptide mimic polymyxin B. Furthermore, it showed a severe defect in motility and an attenuated phenotype in a competitive mouse infection experiment, where the DeltawbaP strain (SKI12) was directly compared to wild-type Salmonella. Nevertheless, the DeltawbaP strain (SKI12) efficiently invaded HeLa cells in vitro and elicited acute intestinal inflammation in streptomycin-pretreated mice. Our experiments prove that the presence of complete LPS is not essential for in vitro invasion or for triggering acute colitis.

The outer core lipopolysaccharide of Salmonella enterica serovar Typhi is required for bacterial entry into epithelial cells

Salmonella enterica serovar Typhi causes typhoid fever in humans. Central to the pathogenicity of serovar Typhi is its capacity to invade intestinal epithelial cells. The role of lipopolysaccharide (LPS) in the invasion process of serovar Typhi is unclear. In this work, we constructed a series of mutants with defined deletions in genes for the synthesis and polymerization of the O antigen (wbaP, wzy, and wzz) and the assembly of the outer core (waaK, waaJ, waaI, waaB, and waaG). The abilities of each mutant to associate with and enter HEp-2 cells and the importance of the O antigen in serum resistance of serovar Typhi were investigated. We demonstrate here that the presence and proper chain length distribution of the O-antigen polysaccharide are essential for serum resistance but not for invasion of epithelial cells. In contrast, the outer core oligosaccharide structure is required for serovar Typhi internalization in HEp-2 cells. We also show that the outer core terminal glucose residue (Glc II) is necessary for efficient entry of serovar Typhi into epithelial cells. The Glc I residue, when it becomes terminal due to a polar insertion in the waaB gene affecting the assembly of the remaining outer core residues, can partially substitute for Glc II to mediate bacterial entry into epithelial cells. Therefore, we conclude that a terminal glucose in the LPS core is a critical residue for bacterial recognition and internalization by epithelial cells.

Penetration of group B streptococci through polarized Madin-Darby canine kidney cells

Group B streptococci (GBS) are one of the major causes of invasive neonatal infection. The pathogenesis of early onset disease is a multistep process. Adhesion of GBS to eucaryotic cells is considered to be an important step for the establishment of infection. Subsequent to adhesion, GBS invade cells and give rise to septicemia and meningitis. To investigate passage of GBS across epithelial cell linings we examined the interaction between bacteria and Madin-Darby canine kidney (MDCK) cells. When grown on permeable support, these cells form a polarized epithelial monolayer with an apical-to-basolateral orientation, which more reflects the in vivo situation compared with conventionally cultured cells. Our results show that GBS are translocated in vacuoles from the apical to the basolateral surface of MDCK cells in a temperature-dependent process. The passage of GBS through the cells is selective with only small numbers of bacteria penetrating in the basolateral-to-apical direction. Transcytosis of GBS starts before decrease in transepithelial resistance of the monolayer. These data suggest a mechanism for traversal of GBS over intact chorioamniotic membranes and from alveoli into the circulation of the fetus.

Association with MDCK epithelial cells by Salmonella typhimurium is reduced during utilization of carbohydrates

Association of Salmonella typhimurium with MDCK epithelial cells in monolayers, represented primarily by intracellular bacteria after 30 min of contact, with centrifugation followed by vigorous washing, was measured during aerobic and anaerobic growth of the bacteria in brain heart infusion broth. Cell association was greatest during a short period in the late log phase of growth under aerobic conditions. At this time, the pH of the growth medium was changing from acid to alkaline and glucose (0.2% initially) was exhausted. Addition of excess glucose (0.5%) to brain heart infusion broth, which was not exhausted before the bacteria entered the stationary phase of growth, in which cell association dropped sharply, resulted in repression of cell association by the bacteria. The repressive effect of glucose on cell association could not be reversed by exogenous cyclic AMP in the bacterial growth medium. Under anaerobic conditions, the effect of glucose on cell association by the bacteria was not as great and the glucose was not exhausted before the bacteria entered the stationary phase. When S. typhimurium was grown in a rich but carbohydrate-free medium, cell association by the bacteria increased earlier in the growth cycle under both aerobic and anaerobic conditions. The addition of glucose and certain other utilizable carbohydrates to this medium caused a repression of cell association by S. typhimurium that was greater under aerobic growth conditions. These results show that cell association by S. typhimurium, which is accompanied by rapid internalization (cell invasion), is the same under aerobic and anaerobic conditions if the bacteria are grown to the log phase in a carbohydrate-free medium. This suggests that prior reports of greater cell invasion by S. typhimurium during anaerobic growth may have arisen from the use of media containing carbohydrates which were found to be more repressive during aerobic growth of the bacteria.

Contact with epithelial cells induces the formation of surface appendages on Salmonella typhimurium

The enteric bacteria Salmonella typhimurium has the ability to invade (enter) nonphagocytic cells. The internalization process occurs as a result of an intimate interaction between the bacteria and the host cell, in which S. typhimurium triggers a cascade of host cell-signaling events leading to the formation of host cell membrane ruffles and bacterial uptake. Using high resolution scanning electron microscopy, we have observed that contact with cultured epithelial cells results in the formation of appendages on the surface of S. typhimurium. The formation of such appendages did not require de novo protein synthesis, and it was transient, since these surface structures were no longer present on bacteria that had initiated the internalization event. Salmonella mutants defective in the transient formation of these surface organelles were unable to enter into cultured epithelial cells, indicating that such structures are required for bacterial internalization.

Morphological and cytoskeletal changes in epithelial cells occur immediately upon interaction with Salmonella typhimurium grown under low-oxygen conditions

Salmonella typhimurium grown under oxygen-limiting conditions were found to enter into, elicit actin filament rearrangement in, and effect morphological changes upon HEp-2 cells within 15 min after infection. Video microscopy revealed that host cell morphological changes associated with entry began within 1 min of productive adherence. Polarized Caco-2 cell morphology was affected 40 s after infection with low-oxygen-grown S. typhimurium. Stationary-phase S. typhimurium did not elicit these phenomena within this time-period even when adherence was enhanced with the afimbial adhesin, AFA-I. Thus, environmental cues regulate S. typhimurium invasion factors, allowing for immediate entry into host cells. Additionally, actin filament rearrangement and morphological changes in the eukaryotic host cell are essential for entry and occur within minutes of infection.

Invasion by Salmonella typhimurium is affected by the direction of flagellar rotation

When grown aerobically, Salmonella typhimurium exhibits a low level of entry into tissue culture cells. We have isolated an S. typhimurium Tn10 mutant which, when grown under aerobic conditions, efficiently invades HEp-2 cells. Sequencing of S. typhimurium DNA adjacent to the site of the Tn10 element showed that the insertion disrupted transcription of the aspartate receptor gene, tar. Polar effects of the transposon on downstream genes also eliminated chemotaxis. Isogenic nonchemotactic (Che-), as well as nonmotile (Mot-) and nonflagellated (Fla-), S. typhimurium strains were examined for their ability to invade HEp-2 cells. "Smooth" swimming Che- mutants (cheA, cheW, cheR, and cheY) were found to possess increased invasiveness for cultured mammalian cells. In contrast, a "tumbly" cheB mutant and Mot- (flagellated) strain were found to have decreased levels of tissue culture invasiveness. A Fla- strain was found to be as invasive as the wild-type strain if centrifugation was used to facilitate contact with the monolayer surface. In addition, the observed hyperinvasiveness of the smooth swimming tar::Tn10 mutant was suppressed when the strain was paralyzed by the introduction of a mot or fla mutation. A murine infection model was used to demonstrate that the mutant invasive phenotypes were also observed in vivo. These data are most consistent with the idea that the rotation and physical orientation of flagella around the bacteria affect the ability of salmonellae to enter host cells.

Distribution of the invA, -B, -C, and -D genes of Salmonella typhimurium among other Salmonella serovars: invA mutants of Salmonella typhi are deficient for entry into mammalian cells

Invasion of intestinal epithelial cells is an essential virulence factor of salmonellae. A group of genes, invABC and invD, that allow Salmonella typhimurium to penetrate cultured epithelial cells have previously been characterized (J. E. Galán and R. Curtiss III, Proc. Natl. Acad. Sci. USA 86:6383-6387, 1989). The distribution of these genes among Salmonella isolates belonging to 37 different species or serovars was investigated by Southern and colony blot hybridization analyses. Regions of high sequence similarity to the invABC genes were present in all Salonella isolates examined, while regions of sequence similarity to the invD gene were present in all but one (S. arizonae) of the isolates tested, with little restriction fragment length polymorphism. Sequences similar to these genes were not detected in strains of Escherichia coli, Yersinia spp., or Shigella spp. invA mutants (unable to express the invABC genes) of several Salmonella species or serovars, including S. typhi, were constructed and examined for their ability to penetrate Henle-407 cells. All mutants were deficient for entry into cultured epithelial cells, indicating that the invABC genes were not only present in these strains but also functional.

Acridine orange stain for determining intracellular enteropathogens in HeLa cells

Green-fluorescent intracellular enteropathogenic bacteria were observed after infected HeLa cell monolayers were stained with acridine orange and counterstained with crystal violet at least 3 h after infection.

HeLa cell invasion by a strain of enteropathogenic Escherichia coli that lacks the O-antigenic polysaccharide

The interaction with HeLa cells of an enteropathogenic Escherichia coli (EPEC) strain and its plasmid-cured derivative strain was examined. An O111:NM EPEC strain B171 harbours a 54 megadalton plasmid (pYR111) necessary for the expression of both localized adherence (LA) to HeLa cells and the O-repeating side chain of the lipopolysaccharide. Under light microscopy, the plasmid-cured derivative strain B171-4 was observed to interact with HeLa cells in a pattern distinct from LA. Transmission electron microscopy showed that the bacteria were internalized by HeLa cells. In contrast, strain B171 induced pedestal-like projections and invaginations of the plasma membrane, but was never completely internalized. A quantitative assay to determine the number of internalized bacteria revealed that strain B171-4 was internalized at levels 30-70-fold higher than those of avirulent E. coli strains. Cytochalasin B reduced the levels of internalization of both strain B171-4 and an enteroinvasive E. coli strain (E11), but did not affect LA by strain B171. These results suggest that EPEC strain B171 may carry a specific chromosomally determined surface factor needed to initiate internalization by HeLa cells. However, a plasmid-determined factor alters the nature of this interaction; the combined effects of the chromosomal and plasmid determinants lead to the characteristic attachment of the bacteria in clusters on the surface of the eukaryotic cell.

Entry and intracellular localization of Brucella spp. in Vero cells: fluorescence and electron microscopy

Vero cells were inoculated with the six species of Brucella (B. abortus, B. melitensis, B. suis, B. neotomae, B. canis, and B. ovis) and examined by fluorescence and electron microscopy. All Brucella spp. were internalized by Vero cells. In all cells except those inoculated with B. canis, the numbers of intracellular brucellae increased with time after inoculation. Intracellular brucellae were first seen within phagosomes and phagolysosomes. Subsequent localization within cisternae of the rough endoplasmic reticulum was seen with all species of Brucella, except B. canis, which was restricted to phagolysosomes. Although rough brucellae were more adherent and entered a greater number of Vero cells, intracellular replication occurred in a larger percentage of cells with smooth rather than with rough brucellae. These results suggest that phagocytosed Brucella spp. are transferred 1) to cisternae of the rough endoplasmic reticulum, where unrestricted bacterial replication takes place; or 2) to phagolysosomes in which Brucella spp. fail to replicate. The various strains of Brucella spp. differ in their ability to induce their own transfer to the rough endoplasmic reticulum.

Penetration and intracellular growth of Brucella abortus in nonphagocytic cells in vitro

In pregnant ruminants, Brucella abortus localizes and replicates within the rough endoplasmic reticulum of trophoblastic epithelial cells. In this study, Vero cells were exposed to B. abortus to investigate its internalization and intracellular growth in nonphagocytic cells. A new double-fluorescence staining procedure to discriminate between extracellular and intracellular bacteria was developed. Studies with the double-fluorescence staining procedure and quantitative bacteriologic culture of disrupted host cells showed that various B. abortus strains replicated within Vero cells, including smooth virulent (strains 2308S and 544), smooth attenuated (strain 19), and rough (strains 45/20 and 2308R) strains. Rough brucellae were more adherent and entered a greater number of Vero cells. Intracellular replication occurred in a larger percentage of cells with smooth virulent (2308S and 544) strains than with smooth attenuated (19) or rough (45/20 and 2308R) strains. Differences in adhesiveness and invasiveness were correlated to hydrophobicity of the organism, as measured by hydrocarbon adherence. Ultrastructurally, intracellular smooth (2308S) and rough (45/20) brucellae were consistently found within cisternae of the rough endoplasmic reticulum and nuclear envelope. The results suggest that transfer to the rough endoplasmic reticulum is the limiting step in the infection of nonphagocytic cells by B. abortus.

The ability of Salmonella to enter mammalian cells is affected by bacterial growth state

We have examined the effect of different growth conditions on the ability of Salmonella to interact with Madin-Darby canine kidney cells. Two growth conditions that affect the expression of Salmonella adherence and invasiveness have been identified. First, bacteria lose their invasiveness in the stationary phase of growth. Second, bacteria growing in oxygen-limited growth conditions are induced for adherence and invasiveness, whereas those growing aerobically are relatively nonadherent and noninvasive. Salmonella from cultures aerated with gas mixtures containing 0% or 1% oxygen were 6- to 70-fold more adherent and invasive than those from cultures aerated with a gas mixture containing 20% oxygen. The Salmonella typhimurium oxrA gene that is required for the anaerobic induction of many proteins is not involved in the regulation of Salmonella invasiveness. We speculate that oxygen limitation might be an environmental cue that triggers the expression of Salmonella invasiveness within the intestinal lumen and other tissues.

Cloning and molecular characterization of genes whose products allow Salmonella typhimurium to penetrate tissue culture cells

Invasion of the intestinal epithelium is thought to be an important step in the pathogenesis of Salmonella infections. Using an in vitro system, we have isolated a genetic locus, inv, that confers to a noninvasive strain of Salmonella typhimurium the ability to penetrate tissue culture cells. Highly virulent S. typhimurium strains carrying inv mutations were defective for entry into Henle-407 cells while remaining unaffected in their ability to attach to cultured cells. When administered perorally to BALB/c mice, inv mutants of S. typhimurium had higher 50% lethal doses (LD50) than their wild-type parent strains. To the contrary, there were no differences in the observed LD50 when strains were administered intraperitoneally. In addition, inv mutants presented decreased ability to colonize the Peyer's patches, the small intestinal wall, and the spleen when administered perorally, although when administered intraperitoneally, they showed no difference in their ability to colonize the spleen compared to the wild-type parent strain.

Adherence of Salmonella typhimurium to small-intestinal enterocytes of the rat

The adherence of radiolabeled Salmonella typhimurium to freshly isolated enterocytes of rats was studied. The results established that type 1 fimbriated strains adhered in significantly higher numbers than did related nonfimbriated strains. Adherence was inhibited by D-mannose and methyl alpha-D-mannoside. Results of kinetic studies indicated that adherence was biphasic; the number of bacteria that adhered per enterocyte remained constant for approximately 20 min and then increased rapidly under the assay conditions. The second phase was associated with structural damage to the enterocytes. The addition of chloramphenicol did not prevent the initial attachment of bacteria to enterocytes but did prevent the second phase. Viable and nonviable bacterial cells adhered to enterocytes, but only viable bacteria were destructive. Freshly isolated enterocytes (trypan blue impermeable) and enterocytes stored overnight (trypan blue permeable) were infected by viable S. typhimurium in a similar manner, suggesting that metabolic activity of the host cell was of less consequence than metabolic activity of the bacterial cells. A model for the role of mannose-sensitive fimbriae as a virulence factor is proposed.

An ultrastructural study of HeLa cell invasion with Salmonella typhi GIFU 10007

Scanning electron micrograph of HeLa S3 monolayered cells, inoculated with viable bacteria of a Salmonella typhi strain GIFU 10007, revealed that the extended microvilli tangled the bacteria within 10 min after inoculation. The micrographs of HeLa cells, at 1 hr after inoculation, indicate the following: shortening of bacterium-attached microvilli, subsiding of tangled bacteria into microvilli bush, and then attachment of bacterial soma to cell surface making the cell membrane depressed. The transmission electron micrographs, at 1 hr after inoculation, demonstrated the findings of interaction between HeLa cell and S. typhi 10007, similar to those observed on scanning electron micrographs. Hair-like fine structures from the soma of challenge organisms were also observed. They were in contact with HeLa cell microvilli and cell membrane. The bacteria were first partially and then totally surrounded by the HeLa cell plasma membrane. One, two, or several bacteria with intact outer membrane were enclosed in intracytoplasmic membrane-bound vacuoles. Fragmented vacuolar membrane was still visible around the intracellularly accumulated bacteria at 24 hr after inoculation. The viable cells of S. typhi 10007 are regarded as internalizing into HeLa cells by a process of endocytosis and to multiply within the membrane-bound vacuoles.

Invasiveness of Salmonella typhi strains in HeLa S3 monolayer cells

The internalization and intracellular multiplication, i.e., the invasiveness, of Salmonella typhi strains recently isolated from typhoid fever patients were confirmed in HeLa cell monolayers. When stained with Giemsa solution, intracellular bacteria were 0.6 X 1.2 micron in size and stained purple, whereas extracellular bacteria associated or not with the HeLa cell surface were 1.0 X 3.0 micron and stained deep blue. Strain GIFU 10007 was internalized into 23% of the HeLa cells within 10 min after inoculation. About 90% of the HeLa cells were infected after 24 hr incubation in kanamycin (KM)-containing medium. Intracellular multiplication of the challenge organism was verified by a large number of intracellular bacteria after 24 hr incubation in KM-containing medium by both light-microscopy of the Giemsa stained preparation and viable counts of intracellular bacteria. The viable counts of strain 10007 showed an increase of more than 40-fold within 24 hr after inoculation, whereas in the four other less or non-infective strains, recovery of viable bacteria was poor or nil. Strains which were highly invasive usually failed to show strong adhesion. The contribution of Vi antigen to the internalization of challenge organisms was not proved. Infective strains, when killed by formalin were still adhesive, but were not internalized. The same strains, when killed by boiling, were neither adhesive nor internalized. From these findings it was concluded that the internalization and multiplication of infective S. typhi strains in cultured HeLa cells should be regarded as an invasion rather than phagocytosis by host cells, and such invasiveness could be an indicator to estimate the virulence of S. typhi strains.

Association with HeLa cells of Campylobacter jejuni and Campylobacter coli isolated from human feces

We developed a rapid in vitro test for determining the association of Campylobacter jejuni and C. coli with HeLa cells. Association was expressed as a weighted mean of the number of bacteria associated with one cell in an association index (AI). The reproducibility of the AI was checked by repeating the test six times, using four strains chosen at random. Means and standard deviations of the means were 7.3 +/- 1.2, 6.8 +/- 0.9, 1.8 +/- 1.2, and 0.1 +/- 0.2. The experimental conditions for which the results are reliable have been standardized. Among 42 strains from human feces, two groups appeared: for 22 nonassociative strains (52%), AI values ranged from 0.0 to 2.1 (mean +/- SD, 0.5 +/- 0.6); for 20 associative strains (48%), AI values ranged from 3.5 to 8.3 (mean +/- SD, 6.2 +/- 1.4). Of these 42 strains, 17 were clinically documented. Diarrhea occurred more frequently in patients infected with associative strains than in those infected with noninvasive strains (7/7 versus 3/10, P = 0.01). Fever also occurred more frequently in patients infected with associative strains (6/7 versus 2/10, P = 0.03). Transmission electron microscopy and viable counts made after killing of extracellular bacteria by gentamicin support the fact that associated Campylobacter spp. are adherent to the cell membrane and are internalized into cytoplasmic vacuoles. The described test seems to be a convenient and rapid method for estimating the pathogenicity of a given strain.

Invasiveness of Aeromonas spp. in relation to biotype, virulence factors, and clinical features

Of 69 fecal isolates of Aeromonas spp., 18 had the ability to invade HEp-2 cells. Invasiveness correlated with biotype; of the 18 invasive strains, 16 were A. sobria and 2 were A. hydrophila. No invasive strains were found among the A. caviae. Of the 18 invasive strains, 13 were enterotoxigenic. Of the enterotoxigenic and invasive strains, 12 were A. sobria, but enterotoxicity was also more common among noninvasive strains of A. sobria. Fucose-resistant hemagglutination was also more common in A. sobria, but invasive strains were equally divided between fucose-resistant hemagglutination and other patterns. Detailed clinical information was available for 27 of the 69 strains. All 15 strains of A. sobria or A. hydrophila associated with diarrhea were enterotoxigenic; 6 of the 10 strains of A. sobria were also invasive. Blood was present in the stool samples of five of the six patients with invasive A. sobria and in none of the patients with noninvasive strains. Although limited, these observations suggest that dysenteric symptoms may be produced by invasive Aeromonas spp.

Activation of polymorphonuclear leukocytes by salmonella

The interaction of polymorphonuclear leukocytes (PMN) with salmonella, as studied by chemiluminescence and phagocytosis, was very different for a number of clinical isolates. Particularly bacteria in the serogroups C1 and E4 deviated from other Salmonella. The differences were observed in the rate of activation, peak value, duration of the chemiluminescence, and in the extent of association and ingestion as studied microscopically. Old laboratory S-strains such as Salmonella typhimurium 395 MS and S. minnesota S99 , which did not associate with the PMN, showed little activation of the PMN, whereas their phagocytosis-sensitive R-mutants induced rapid activation, high peak values, and short duration of the chemiluminescence. Certain isolates belonging to the C1/E4 group induced intermediate types of reactions. The kinetics of the activation was related to the physicochemical surface properties of the bacteria. Heating the bacteria at 70 degrees C for 45 min enhanced the activation of PMN by the S-type strains conspicuously but in different ways, whereas that of R-mutants was delayed. Different clinical isolates of salmonella have shown different physico-chemical surfaces, liability to phagocytosis by PMNs and different degrees of eliciting inflammatory mediators from PMNs in vitro. The results indicate that the C1/E4 group of Salmonella has pathogenicity mechanisms different from most salmonellae.

Vero cell invasiveness of Proteus mirabilis

Vero cell invasiveness was studied for a group of Proteus mirabilis strains isolated from the urinary tract and feces and for a limited group of urinary isolates of Escherichia coli. Experimental conditions affecting this invasiveness were studied. All of the P. mirabilis strains tested were capable of cell invasion, whereas none of the E. coli strains was. Correlation between the hemolytic activity of the P. mirabilis strains and their invasive ability suggested that the bacterial hemolysin may be involved in the invasion process. Other experimental evidence supporting this hypothesis is discussed. The differences in the invasive capacities of P. mirabilis and of E. coli may be important for the apparent differences in the pathogenesis of urinary tract infection by both species.

Physico-chemical surface properties of Shigella sonnei

In partition in two-phase systems as well as in hydrophobic and ion exchange chromatography a number of Sh. sonnei isolates with different phage patterns showed similar physico-chemical surface properties. All the isolates displayed hydrophobicity and a small negative charge. In this way the shigellae were different from salmonella S-strains and E. coli strains associated with infantile diarrhoea which have shown hydrophilic properties and nearly no charge. They were also different from E. coli strains with typical K-antigen often grown from patients with septicemia and upper urinary tract infection, which have shown hydrophilic properties and a great negative charge. However, the Sh. sonnei isolates were similar to E. coli strains belonging to certain serogroups which have been grown from patients with dysentery-like diseases. The results indicate that physico-chemical surface properties are related among enterobacteria belonging to different genera which have been grown from similar types of infections.

Haemagglutinating, adhesive and physico-chemical surface properties of different Yersinia enterocolitica and Yersinia enterocolitica-like bacteria

Fourteen strains of Yersinia enterocolitica, two strains of Yersinia kristensenii and one strain of Yersinia frederiksenii were investigated for haemagglutination, association with cultured cells, motility and physico-chemical surface properties. Three of the Y. enterocolitica strains, both Y. kristensenii and the Y. frederiksenii strain caused haemagglutination. Y. enterocolitica O-serotype 3 had the greatest tendency to associate with cultured cells. No correlation was seen between association and haemagglutination or motility. Bacterial cytotoxicity towards cultured cells was lost and haemagglutination acquired after repeated subcultivation. haemagglutinating strains were more hydrophobic than non-haemagglutinating strains. These results indicate that the bacterial surface structures, probably fimbriae, conferring bacterial haemagglutinating properties, are hydrophobic.

Enhanced phagocytic response of macrophages to bacteria by physical impact caused by bacterial motility or centrifugation

The mechanism of enhanced phagocytic and chemiluminescent responses of macrophages caused by bacterial motility (T. Tomita, E. Blumenstock, and S. Kanegasaki, Infect. Immun. 32:1242, 1981) was studied. Both responses increased up to a certain level with an increased number of motile bacteria, such as Salmonella typhimurium, Escherichia coli, or Pseudomonas aeruginosa, added. In contrast, only a slight increase was observed with the motility (mot) mutants of these bacteria, even when 4,000 bacteria per single macrophage were added. If nonmotile bacteria were centrifuged together with a monolayer culture of macrophages, the number of bacteria ingested per macrophage increased dramatically. This phenomenon was not observed in the presence of cytochalasin B or at a low temperature, and about half of the associated bacteria were killed within 30 min of prolonged incubation, indicating that the bacteria were not simply embedded on the macrophage surface. An observed biphasic increase of ingestion with an increase in centrifugal force suggested the existence of a threshold velocity for efficient phagocytosis. The minimum centrifugal force required for maximal response was determined under the conditions in which equalized collision frequency between bacteria and macrophages was maintained when different centrifugal forces were employed. From the value obtained (5 x g), the required rate of movement was calculated as approximately 2.5 microns/s, supposing that the bacterium is spherical and has a 1-micron radius. This value is much lower than the velocity of movement of motile bacteria (20 to 50 microns/s). The results indicate that physical impact caused by bacterial motility is enough to induce a high response of macrophages.

Anionic sites on the envelope of Salmonella typhimurium mapped with cationized ferritin

Binding of either ferritin (F) or cationized ferritin (CF) was employed to indicate the surface charge of the envelope of mainly two Salmonella typhimurium strains (395 MR10, a Rd-mutant, and LT2-M1, a UDP-galactose-4-epimerase-less mutant). Lowering the pH from 7 to 4 decreased binding of CF, but increased binding of F. At low concentrations, the distribution of CF on S. typhimurium 395 MR10 was in general random, with individual ferritin molecules often forming clusters of two or three particles. At ionic strengths of 0.25M NaCl, ferritin produced distinctive, larger clusters at relatively few sites (10-50/cell). Addition of galactose to cultures of growing S. typhimurium, LT2-M1 reduced the binding of CF in 1-10 min, and numerous ferritin-free areas became visible. Possibly this is caused by a pluri-focal reduction in the negative cell surface charge that was generated at the multiple sites of export of new, smooth-type lipopolysaccharide, which either exhibits lesser charge or masks a preexisting surface charge. Dividing cells may show unequal charges on the prospective daughter cells, and the difference in the capacity for ferritin adsorption of both daughter cells is sharply separated at the division site.

Adherence of Acinetobacter calcoaceticus RAG-1 to human epithelial cells and to hexadecane

The ability of Acinetobacter calcoaceticus RAG-1 to adhere to human epithelial cells was investigated and compared with its ability to adhere to a test hydrocarbon (hexadecane). RAG-1, a microorganism originally isolated for growth on hydrocarbon, adhered to epithelial cells when grown under conditions which promote its adherence to hexadecane; similarly, RAG-1 cells adhered poorly to epithelial cells when grown under conditions which cause the cells to possess low affinity towards hexadecane. A mutant derived from RAG-1, MR-481, deficient in its ability to adhere to hydrocarbon, was similarly unable to adhere to epithelial cells. RAG-1 adherence to epithelial cells was not blocked by a number of sugars tested. Streptococcus pyogenes, whose adherence to epithelial cells has been previously attributed to hydrophobic interactions, was also able to adhere to hexadecane. Results suggest that hydrophobic interactions mediate adherence of the strains studied to both epithelial cells and hydrocarbon.

Phagocytic and chemiluminescent responses of mouse peritoneal macrophages to living and killed Salmonella typhimurium and other bacteria

In the presence of luminol, resident as well as thioglycolate-induced and immunized macrophages emitted chemiluminescence more efficiently when the cells were exposed to living Salmonella typhimurium than when they were exposed to the same bacterium killed by ultraviolet light or heat. This phenomenon was observed whether or not the bacterium was opsonized. The different response to living and killed bacteria was also found with Escherichia coli, Pseudomonas aeruginosa, Proteus morganii, and Enterobacter aerogenes, but not with Shigella sonnei, Klebsiella pneumoniae, and Propionibacterium acnes. The results suggest that macrophages respond better to living, motile bacteria than to nonmotile or killed bacteria. The experimental results obtained with motility mutants of S. typhimurium, E. coli, and P. aeruginosa confirm that macrophages exposed to the motile bacteria emit chemiluminescence more efficiently and ingest the motile bacteria at a much faster rate than the nonmotile bacteria.

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.

Association with HeLa cells of LPS mutants of Salmonella typhimurium and Salmonella minnesota in relation to their physicochemical surface properties

Different LPS mutants of Salmonella typhimurium and Salmonella minnesota have been investigated with respect to (1) their tendency to associate with HeLa cell monolayers, and (2) their physicochemical surface properties. Aqueous biphasic partitioning, hydrophobic interaction chromatography, and ion exchange chromatography have been used to characterize the bacterial cell surface properties with respect to charge and hydrophobicity. Liability to hydrophobic interaction was defined either by the change of partition in a dextran-polyethylene-glycol (PEG) system by the addition of PEG-palmitate (P-PEG), or by the elution pattern from Octyl-Sepharose. Accordingly, charge was asssessed by the effect of positively charged trimethylamino-PEG (TMA-PEG) on the partition, and by the elution from DEAE-Sephacel. Bacterial being negatively charged and liable to hydrophobic interaction had the highest tendency to associate with HeLa cells. In some cases the methods for surface analysis gave conflicting results on charge and/or liability to hydrophobic interaction of the same LPS mutant. Possible reasons for these differences and the role of bacterial cell surface structures contributing to physicochemical character are discussed.

Attachment of bacteria to mammalian surfaces

Much attention has been devoted to the study of bacterial adherence to mammalian surfaces in vitro during the past several years. Some in vivo evidence also suggests that this process may indeed be an integral part of the pathogenesis of colonization and certain infections. The biochemical basis of attachment and definition of the actual receptor sites involved are just starting to become known and seem to be different amongst individual bacteria genera. However, pili may mediate attachment of a variety of gram-negative organisms to receptor cells, and streptococcal lipoteichoic acids probably serve a similar function. Some recent study methods and results in this field are reviewed.

Surface properties of Chlamydia psittaci

The surface properties of elementary bodies of Chlamydia psittaci were investigated by diethylaminoethyl-Sephadex chromatography, cytophoresis, partitioning in an aqueous polymer two-phase system, and hydrophobic interaction chromatography of the organism. The surface of C. psittaci was found to be hydrophobic and negatively charged at pH 7 and to have an isoelectric point of about pH 5. Reagents which block free carboxyl groups altered the surface charge of C. psittaci and caused the organism to agglomerate. The possible significance of hydrophobicity and surface charge on the ingestion of C. psittaci by host cells is discussed.

Shigella infection of Henle intestinal epithelial cells: role of the bacterium

Epithelial cell infection by Shigella flexneri 2a was studied in an in vitro model system. Using the Henle 407 human intestinal epithelial cell line as host cells, a standardized experimental protocol which allowed quantitative measurement of infection was developed. Intravellular residence of infecting organisms was confirmed by indirect fluorescent-antibody staining of unfixed and methanol-fixed (Henle 407) cells and by quantitative bacteriological culture of disrupted host cells after infection. The process of shigella entry into cells was evaluated by chemical or physical modulation of the bacterium under controlled experimental conditions. Shigella were subjected to mild heat, ultraviolet radiation aminoglycoside antibiotics, and immunoglobulins raised against S. flexneri 2a. The data show that heat-stable antigens on the bacterial surface are not solely responsible for infectivity of S. flexneri 2a. Furthermore, it was shown that physiological and synthetic functions of shigellae are required for entry into host cells.

Ultrastructural studies on the interaction between Salmonella typhimurium 395 M and HeLa cells

The interaction of Salmonella typhimurium 395 MS and its rough Rd-mutant 395 MR10 with HeLa cells was studied by transmission and scanning electron microscopy. The bacteria attached to central as well as more marginal positions of the HeLa cell surface. Bacteria associated preferentially to HeLa cells with a relatively low number of microvilli, in which they often were entangled. Bacteria attached to the cell border were sometimes surrounded by membrane folds, possibly as a response to their attachment. Infected cells had longer and more slender microvilli compared with noninfected cells. Some parts of the attached bacteria were in close contact with the HeLa cell membrane, whereas other parts were separated from the latter by a gap. Bacteria adhered preferentially to microvilli without obvious membrane damage. Most of the intracellular bacteria were surrounded by a membrane, often appearing as a vacuole, which sometimes contained more than one bacterium. Intracellular bacteria seemed to be morphologically intact. We propose that S. typhimurium enter HeLa cells by a process of phagocytosis.

Intestinal distribution of Vibrio cholerae in orally infected infant mice: kinetics of recovery of radiolabel and viable cells

Kinetics of distribution of Vibrio cholerae in the gastrointestinal tract of orally challenged infant mice were examined by determining recovery of input dose from the whole gut and from individual segments of stomach, upper bowel, and lower bowel. The strains studied were 569B, CA401, and VB12 (a rough CA401). Recovery was determined as a percentage of either input radiolabel using 35S-labeled cells or input colony-forming units. We found clearance of radiolabel and viable cells from the stomach into the intestines by 2 h. Early whole-gut clearance of label was greater for 569B and heat-killed CA401 than for CA401, VB12, or Formalinized CA401. At early times postchallenge, significant differences occurred between strains in the upper bowel, with greater recovery of label and viable cells for CA401 than for 569B or VB12. Beginning at 8 h postchallenge, radiolabel accumulated in the lower bowel with all experimental groups except CA401-challenged mice, where diarrhea was noted and label disappeared from the intestines. In vitro evaluation of mucosal association of these strains with bowel sections was also done. CA401 and VB12 associated to a greater extent than 569B or heat-killed or Formalin-killed CA401.

Role of phagocytosis in mouse virulence of Salmonella typhimurium recombinants with O antigen 6,7 or 4,12

The quality of lipopolysaccharide has previously been shown to influence the mouse virulence of Salmonella so that strains with O antigen 4,12 were more virulent than their O-9,12 sister strains. Immunosuppression did not alter this O-antigen-dependent difference in virulence. I have now constructed smooth O-4,12 and O-6,7 sister hybrid strains of Salmonella typhimurium. No other phenotypic differences were found between these strains; they were all "common antigen" positive. In intraperitoneal infection, the O-4,12 strains were more mouse virulent than their O-6,7 sisters. The difference in virulence correlated with a difference in clearance rates; the O-6,7 hybrids were removed from the blood more rapidly than their O-4,12 sisters. No natural bactericidal antibodies were found in the sera of the mice.

Association of some enterobacteria with the intestinal mucosa of mouse in relation to their partition in aqueous polymer two-phase systems

The association of enterobacteria with mouse intestinal mucosa has been investigated by pumping heat-killed, radioactively-labelled bacteria through the gut lumen in vitro. Approximately 20 cm of the small intestine proximal to the ileo-caecal valve was rinsed, excised and maintained in an organ bath. By using two different bacteria labelled with different radioactive isotopes, the relative association of the two bacterial pumped through the same piece of gut was determined. Cross-labelling showed that choice of isotope did not affect the association. Salmonella typhimurium 395 MR10 was used as reference and the other bacteria investigated related to it. S. typhimurium MR10 and Escherichia coli O 14 K7, which are relatively lipophilic, showed greater association than S. typhimurium 395 MS and E. coli O 111 K58, which are more hydrophilic. Prolonged incubation of bacteria with the length of intestine in vitro leading to damage of the brush border of the mucosal epithelium enhanced the association of the bacteria. These data suggest that similar physico-chemical surface properies govern the association certain enterobacteria to the intestinal mucosa as in phagocytosis with professional phagocytes.

Endocytosis of Salmonella typhimurium 395 MS and MR10 by HeLa cells

Monolayer of HeLa cells were examined for their ability to endocytose Salmonella typhimurium 395 MS (wild) and MR10 (chemotype RD). Monolayers treated with the glycolytic inhibitors iodoacetic acid (IAA) or N-ethylmaleimide (NEM) or the respiratory inhibitor sodium azide (NaN3) or cytochalasin B (CB) were incubated with S. typhimurium. The numbers of cell-associated (intracellular plus cell-membrane attached extracellular) and intracellular bacteria were determined by viable counts, together with the HeLa cell ATP levels. IAA and NEM at concentrations 10(-4)M and 10(-3)M decreased significantly the number of intracellular MR10 and the cellular ATP levels, but did not influence significantly the total number of cell-associated bacteria except for 10(-3)M IAA which slightly increased the association. On the other hand, NaN3 at concentrations 10(-4)M and 10(-3)M did not affect the number of associated or intracellular bacteria, or the cellular ATP levels. CB at concentrations of 5, 10 and 20 microgram/ml increased the number of associated bacteria, decreased the number of intracellular bacteria and caused a small decrease in cellular ATP levels. Thus, HeLa cells may internalize S. typhimurium by an energy-requiring, glycolysis-dependent process. CB had a dose-dependent inhibitory effect on the internalization without influencing significantly the HeLa cell ATP levels. This indicates that CB might affect the internalization process by some means other than decreasing the ATP content.

Infection of HeLa cells with Salmonella typhimurium 395 MS and MR10 bacteria

After interaction with HeLa cells cultured in vitro, the fraction of adhering extracellular and that of internalized smooth Salmonella typhimurium 395 MS and rough 395 MR10 have been determined by two different techniques. (i) By using the indirect fluorescent-antibody technique on unfixed and acetone-fixed HeLa cell preparations, intracellular bacteria were considered to become stained only after acetone fixation. (ii) Based on the assumption that gentamicin affects only extracellular bacteria, disintegration of the infected HeLa cells and viable count allowed the determination of internalized bacteria. Both techniques showed that MS as well as MR10 bacteria gained intracellular access, the fraction of MR10 cells doing so being much greater. The net increase in the intracellular bacterial population was small within 3 h of incubation.

Rough mutant of Shigella flexneri 2a that penetrates tissue culture cells but does not evoke keratoconjunctivitis in guinea pigs

A rough mutant, designated 5503-01, has been isolated from a virulent strain of Shigella flexneri 2a 5503. Strain 5503-01 produced smooth opaque colonies, whereas its parent strain produced characteristic green-gold translucent ones. Characterization of 5503-01 by agglutination tests, rhamnose content, and sensitivity spectra to "rough-specific" phages revealed that it had lost the specific somatic antigens. When 5503-01 was used to infect HeLa or L cells, it penetrated the cells and multiplied within the cytoplasm. On the other hand, it could not evoke keratoconjunctivitis in guinea pigs. The properties of this strain were remarkably stable against serial passages and preservation for a long period. The presence of a rough mutant with the ability to penetrate tissue culture cells suggest that specific O antigen is not of significance in the early step of the invasive process of shigellosis.