FINE STRUCTURE OF LISTERIA MONOCYTOGENES

Entropy-driven translocation of disordered proteins through the Gram-positive bacterial cell wall

In Gram-positive bacteria, a thick cross-linked cell wall separates the membrane from the extracellular space. Some surface-exposed proteins, such as the Listeria monocytogenes actin nucleation-promoting factor ActA, remain associated with the bacterial membrane but somehow thread through tens of nanometres of cell wall to expose their amino terminus to the exterior. Here, we report that entropy enables the translocation of disordered transmembrane proteins through the Gram-positive cell wall. We build a physical model, which predicts that the entropic constraint imposed by a thin periplasm is sufficient to drive the translocation of an intrinsically disordered protein such as ActA across a porous barrier similar to a peptidoglycan cell wall. We experimentally validate our model and show that ActA translocation depends on the cell-envelope dimensions and disordered-protein length, and that translocation is reversible. We also show that disordered regions of eukaryotic proteins can translocate Gram-positive cell walls via entropy. We propose that entropic forces are sufficient to drive the translocation of specific proteins to the outer surface.

Proteomic Characterization of Antibiotic Resistance in Listeria and Production of Antimicrobial and Virulence Factors

Some Listeria species are important human and animal pathogens that can be found in contaminated food and produce a variety of virulence factors involved in their pathogenicity. Listeria strains exhibiting multidrug resistance are known to be progressively increasing and that is why continuous monitoring is needed. Effective therapy against pathogenic Listeria requires identification of the bacterial strain involved, as well as determining its virulence factors, such as antibiotic resistance and sensitivity. The present study describes the use of liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) to do a global shotgun proteomics characterization for pathogenic Listeria species. This method allowed the identification of a total of 2990 non-redundant peptides, representing 2727 proteins. Furthermore, 395 of the peptides correspond to proteins that play a direct role in Listeria pathogenicity; they were identified as virulence factors, toxins and anti-toxins, or associated with either antibiotics (involved in antibiotic-related compounds production or resistance) or resistance to toxic substances. The proteomic repository obtained here can be the base for further research into pathogenic Listeria species and facilitate the development of novel therapeutics for these pathogens.

Two-enzyme systems for glycolipid and polyglycerolphosphate lipoteichoic acid synthesis in Listeria monocytogenes

Lipoteichoic acid (LTA) is an important cell wall polymer in Gram-positive bacteria and often consists a polyglycerolphosphate backbone chain that is linked to the membrane by a glycolipid. In Listeria monocytogenes this glycolipid is Gal-Glc-DAG or Gal-Ptd-6Glc-DAG. Using a bioinformatics approach, we have identified L. monocytogenes genes predicted to be involved in glycolipid (lmo2555 and lmo2554) and LTA backbone (lmo0644 and lmo0927) synthesis. LTA and glycolipid analysis of wild-type and mutant strains confirmed the function of Lmo2555 and Lmo2554 as glycosyltransferases required for the formation of Glc-DAG and Gal-Glc-DAG. Deletion of a third gene, lmo2553, located in the same operon resulted in the production of LTA with an altered structure. lmo0927 and lmo0644 encode proteins with high similarity to the staphylococcal LTA synthase LtaS, which is responsible for polyglycerolphosphate backbone synthesis. We show that both proteins are involved in LTA synthesis. Our data support a model whereby Lmo0644 acts as an LTA primase LtaP and transfers the initial glycerolphosphate onto the glycolipid anchor, and Lmo0927 functions as LTA synthase LtaS, which extends the glycerolphosphate backbone chain. Inactivation of LtaS leads to severe growth and cell division defects, underscoring the pivotal role of LTA in this Gram-positive pathogen.

MICROMORPHOLOGY OF DERMATOPHILUS CONGOLENSIS

Gordon, Morris A. (Division of Laboratories and Research, New York State Department of Health, Albany), and Mercedes R. Edwards. Micromorphology of Dermatophilus congolensis. J. Bacteriol. 86:1101-1115. 1963.-As seen in electron micrographs of thin sections, Dermatophilus congolensis is a holocarpic actinomycete that fragments, after formation of septa in several planes, into Sarcina-like packets and then into individual cocci. Release of coccal forms from the filaments and packets is by dissolution of a capsular matrix, which is a product of degradation of the cell wall. The plasmalemma is a "unit membrane." Regularly occurring plasmalemmosomes ("onion bodies") of uniform structure are apparently related to septum formation. A typical bacterial nucleoid is seen in most sections, and ribosomes are scattered throughout the cytoplasm. Specimens for electron microscopy were prepared by a modification of Kellenberger's method.

ELECTRON MICROSCOPE OBSERVATIONS ON INTACT CELLS, PROTOPLASTS, AND THE CYTOPLASMIC MEMBRANE OF BACILLUS STEAROTHERMOPHILUS

Abram, Dinah (Purdue University, Lafayette, Ind.). Electron microscope observations on intact cells, protoplasts, and the cytoplasmic membrane of Bacillus stearothermophilus. J. Bacteriol. 89:855-873. 1965.-Negatively stained preparations of protoplasts and fragments of cytoplasmic membranes from cells of Bacillus stearothermophilus ruptured by treatment with sonic oscillation, partial lysis with lysozyme, autolysis, or phage infection were examined electron microscopically. Specimens of intact cells also were examined by the same technique. The following structural details were revealed. Intact or nearly intact, partially swollen, elongated protoplasts and their ghosts have a characteristic differentiated surface texture and can easily be distinguished from the cell wall. Infoldings of the cytoplasmic membrane can be observed in these protoplasts, to which flagella are attached; the latter originate via hooks from "basal structures" that are in close association with the cytoplasmic membrane or part of it. Abundant intracytoplasmic membranous elements, which appear to be tubular or vesicular, can be seen in whole cells of three of the strains studied. The fine structure of the cytoplasmic membrane and probably that of its intracytoplasmic infoldings was observed on flattened and folded membrane fragments, one layer thick. Structural units, roughly spherical, 65 to 85 A in diameter, were present on one side of the cytoplasmic membrane, facing the cytoplasm. They were attached loosely to the membrane by fine stalks, 40 to 60 A long, and were easily detached, probably leaving the stalks behind them on the membrane. While the greater stability of membranes from thermophiles made this study of the fine structure possible, the structural units described were demonstrated also on cytoplasmic membranes from mesophiles.

CHEMICAL AND IMMUNOLOGICAL COMPOSITION OF SURFACE STRUCTURES OF LISTERIA MONOCYTOGENES

Osebold, John W. (University of California, Davis), Ole Aalund, and Clarence E. Chrisp. Chemical and immunological composition of surface structures of Listeria monocytogenes. J. Bacteriol. 89:84-88. 1965.-A proteinlike surface substance was demonstrated on Listeria monocytogenes when an explanation was sought for the inagglutinability of some somatic antigens. The serological behavior of live bacteria and organisms subjected to heat, formalin, and trypsin was compared. The agglutination-inhibiting phenomenon was most pronounced with heat-killed (100 C) antigens. Trypsinization eliminated inagglutinability and increased sensitivity. Substances released by the enzyme had an ultraviolet-absorption peak at 260 mmu and showed a spot on paper chromatograms compatible with polypeptide. Inagglutinable cells combined with antibody because they could readily absorb antibodies from serum. After reaction with anti-Listeria serum, inagglutinable cells could be agglutinated by the addition of antiglobulin serum. It was hypothesized that heat inactivation of cells denatured the proteinaceous surface layer which interferred with the formation of a visible agglutination product but did not eliminate antigen-antibody reaction.

OBSERVATIONS ON THE STRUCTURE OF RHODOSPIRILLUM MOLISCHIANUM

The lamellae of the bacterium Rhodospirillum molischianum originate as extensions of the cytoplasmic membrane into the cytoplasm of the cell. Initially, these extensions are narrow folds and occur independently of one another. The first lamellae to appear average about 80 A in width, representing one side of the infolded cytoplasmic membrane, or 160 A when the two sides of the fold are closely appressed. The 160-A lamellae increase in number and may associate to form larger lamellae, which represent varying degrees of association between adjacent folds. Later, the space within each fold increases; the two appressed regions of the cytoplasmic membrane in each fold separate to form distinct invaginations, and the lamellae observed at this stage are formed by an association of the sides of adjacent invaginations.

Diagnostic ultrastructure of Listeria monocytogenes in human central nervous tissue

The pathogenesis of Listeria monocytogenes infection of the human central nervous system is poorly understood and ultrastructural aspects are not documented. A case of fatal human L. monocytogenes meningoencephalitis is described, in which diagnosis was confirmed by blood culture and in which special staining (Brown's) showed slender gram-positive bacilli in inflamed areas of the brainstem at autopsy. Electron microscopy of blocks rescued from formalin revealed rods, up to 2.5 microns long x 0.4 microns in diameter, with gram-positive type cell walls and distinctive conic ends, the latter being apparent in axial section only. The organism was either free within the cytoplasm or within endosomes or phagosomes of macrophages, smooth muscle cells, and endothelial cells. In one instance, one was seen adhering to the luminal aspect of the vascular endothelium. Central nervous system parenchymal cell infection was suspected but not confirmed. Intra-cytoplasmic bacteria were surrounded by an electron-transparent halo and, beyond that, a cloud of host-derived microfilaments, as previously described by others in vitro and in animals. The morphology of L. monocytogenes is compared with that of other intracellular pathogens and is found to be distinctive, suggesting the value of ultrastructural study in diagnosis. Such work could also help to solve outstanding pathogenetic questions of central nervous system invasion and spread.

Trichophyton rubrum invasion of human hair apparatus in tinea capitis and tinea barbae: light- and electron microscopic study

We have previously reported morphological changes of Trichophyton violaceum and Microsporum canis in hair apparatuses in tinea capitis. To investigate the morphology of Trichophyton rubrum in the human hair apparatus, two cases of tinea capitis and one case of tinea barbae were examined by light- and electron microscopy. The fungal elements, which were located in the lower keratogenous zone, showed non-septate hyphae in the outer part of the hair cortex. With the upward development of the hair layers, some hyphae invaded the keratinized hair cuticle and keratinized inner root sheath and were transformed into arthrospores. Some hyphae remaining in the hair cortex were also transformed into arthrospores, while other hyphae in the hair cortex did not survive, but degenerated. In T. rubrum hair infection, there is a distinct relationship between the morphological changes of the fungi and the hair cell differentiation as seen in T. violaceum and M. canis infections. However, T. rubrum displays unique morphological changes, which are different from those of T. violaceum and M. canis, in hair apparatuses.

Fine structure of Methanospirillum hungatii

The fine structure of Methanospirillum hungatii was studied by electron microscopy. The topography of the cell wall and the mechanism of cell division are not typical of gram-positive or gram-negative bacteria. A novel architectural arrangement of cells in continuous spiral filaments is described. Filamentous cells are connected by spacers and enclosed within a rigid outer envelope. The unique ultrastructural features of cells and cell spacers were examined.

Effect of lysozyme treatment on cell wall ultrastructure in Sarcina flava

Treatment of Sarcina flava NCTC 7503 cells with lysozyme resulted in incomplete removal of the cell wall and imparted a layered appearance to it.

Fate of listeria monocytogenes in normal rabbit serum

Viability of Listeria monocytogenes suspended in either normal rabbit serum or listericidal factor is significantly reduced before cell lysis. In normal serum Listeria populations are initially reduced by clumping of viable bacteria, and the cells within these clumps are morphologically altered and killed by the listericidal factor in serum. This clumping is complement independent and does not occur after suspension in purified listericidal factor. In electron micrographs, densely stained amorphous material was seen surrounding clumped cells suspended in serum. Initial damage to Listeria suspended in either serum or listericidal factor appears to be confined to plasma and mesosomal membranes. The mesosomes are rapidly disorganized, and the plasma membrane becomes crenated and separates from the cell wall. Subsequently, the fibrillar organization of the nuclear region disrupts, and the cytoplasm increases in granularity. Studies utilizing Listeria protoplasts provided evidence in support of the plasma membrane as the site of listericidal action. Cell death may be related to loss in enzyme function after crenation of the plasma membrane and mesosomal dissolution.

Fine structure of Thermus aquaticus, an extreme thermophile

Electron microscopic studies using thin sections revealed that Thermus aquaticus has a structure similar to that of most other gram-negative bacteria. The cell envelope is tripartite: plasma membrane, thin middle layer, and a thicker and irregular outer layer. The outer layer appears to be joined to the plasma membrane by a series of connections and, when seen in tangential section, the outer layer appears as a series of parallel bands. The cell division mechanism resembles that of typical gram-negative bacteria. Large spherical bodies designated "rotund bodies" are formed as a result of the association of a number of separate cells. In this association the outer envelope layers of the cells fuse and pull away from the middle layer. The rotund body thus appears as a series of rods, usually lying in parallel around the periphery of the sphere, completely connected by means of the fused outer layer.

Bacterial growth and the cell envelope

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Comparative ultrastructure of selected aerobic spore-forming bacteria: a freeze-etching study

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Further studies of the morphology of Treponema pallidum under the electron microscope

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Electron microscopic examination of Corynebacterium ovis

Corynebacterium ovis (C. pseudotuberculosis) was examined by electron microscopy after being subjected to various methods of fixation. The organism exhibited a fine structure similar to other corynebacterial species in the appearance of its cell wall, plasma membrane, nuclear apparatus, cytoplasmic matrix, wealth and complexity of intracytoplasmic membrane systems, and polyphosphate granules. An outstanding structural feature was the existence of an electron-dense, floccular layer external to the cell wall which both ligroin and acetone-methanol extractions demonstrated to be the previously postulated surface lipid of this organism. The only variations in structure evident between virulent and attenuated strains was a quantitative difference in the thickness and appearance of the surface lipid. The observation of this layer provided a basis for explaining the surface properties of C. ovis, with particular respect to its clumping capacity in suspension, the waxiness of its growth on solid media, and its ability to grow as a pellicle on suitable liquid media. The variation in the visible amount of surface lipid between the virulent and avirulent strains adequately explained the divergence of these three surface properties between the strains.

Fractionation and characterization of the plasma and mesosome membrane of Listeria monocytogenes

Protoplasts of Listeria monocytogenes strain 42 were fractionated after control lysis on a Ficoll (a polysucrose) density gradient. Visually, five zones could be recognized in the gradient. The first one was composed of amorphous cytoplasmic solutes (fraction 1a) and a mixture of particles (fraction 1b). These were: (i) light particles that were lipase-sensitive and composed of six subunits and (ii) heavy particles, sensitive to ribonuclease and devoid of fine structure. The second zone consisted of tubules and vesicles still harboring cytoplasmic components (fraction 2), whereas the third zone contained only empty vesicles and protoplast ghosts (fraction 3). The material congregating into the fourth zone was morphologically identical to that of the third (fraction 3a). The fifth and heaviest zone contained a mixture of (i) particles without any substructure and (ii) partly lysed protoplasts (fraction 4). Fractions 1b and 4 were the richest in nucleic acids (ribonucleic acid, 11.4 and 9.4%, respectively; deoxyribonucleic acid, 5.1 and 4.8%, respectively), whereas fraction 1b had the highest protein contents (74.6%). Phospholipids were mainly found in fractions 2 and 3. Except for fraction 1, all materials contained significant amounts of protein-bound phosphorus. The main concentrations of four enzymes were: glucose-6-phosphate dehydrogenase (fraction 1a); adenosine triphosphatase and reduced nicotinamide adenine diphosphate oxidase (fraction 3); nitro blue tetrazolium chloride reductase (fraction 2). Fractionation of strain 42 after addition of (32)P during the mid-log phase of growth revealed that the radio-activity was mainly detected in fraction 1b, when growth in the presence of the marker was allowed for 10 min, and in fraction 2, when growth was allowed for 90 min. The vesicles of fraction 2, often tubular, are probably of mesosomal origin, whereas those of fraction 3, which are always spherical, represent, most likely, the bulk of the cell plasma membrane. Our data showed slight chemical differences between these two fractions, but the differences in enzymatic activities and lipid-phosphorus incorporation during long pulse experiments were most dramatic.

ULTRASTRUCTURE OF THE THERMOPHILIC BLUE-GREEN ALGA, SYNECHOCOCCUS LIVIDUS COPELAND(1)

The ultrastructure of Synechococcus lividus Copeland, a thermophilic blue-green alga, was studied in thin sections. The cell envelope reveals striking similarities with that of some gram-negative bacteria. In contrast to bacteria and to many other species of blue-green algae, ribosomes are predominantly found in the central nuclear region and appear to be associated with the DNA fibrils. Thylakoids (photo-synthetic lamellae) are arranged as concentric shells, around the nuclear equivalent, lying nearly parallel to one another and to the plasma membrane. Both plasma and thylakoidal membranes, as described by other authors for different Cyanophyceae, are of the unit membrane dimension and morphology. Various types of intracellular inclusions are found: (1) Lipid inclusions, located in the cytoplasm are similar to the osmiophilic globules of higher plant chloroplasts. (2) Polyphosphate inclusions (or volutin) resembling those of other species are generally found at the cell poles but within the nuclear region. (3) Polyhedral inclusions also located in the nuclear region are clearly recognized to be different from the polyphosphate bodies, but their function remains unknown.

Isolation, composition, and structure of membrane of Listeria monocytogenes

The plasma membrane of Listeria monocytogenes strain 42 was prepared by osmotic lysis of protoplasts with tris(hydroxymethyl)aminomethane (Tris) buffer, pH 8.2, containing MgCl(2) and glucose, followed by washing with NaCl and MgCl(2) in Tris buffer. Electron microscopy showed that the preparation was not contaminated with cytoplasmic material. The membrane preparation was composed of 55 to 60% protein, 1.5% ribonucleic acid, 0.1% deoxyribonucleic acid, 1.3 to 2.3% carbohydrate, 0.17 to 0.38% amino sugar, 0.2 to 0.4% rhamnose, 3.5 to 4.0% phosphorus, 10.5 to 12.0% nitrogen, and 30 to 35% lipid. Amino acid composition of the washed membrane showed some variation from that of the whole cells. Sulfur-containing amino acids were not present in the membrane hydrolysate. The membrane carbohydrate contained glucose, galactose, ribose, and arabinose. The membrane lipid was 80 to 85% phospholipid and 15 to 20% neutral lipid. The lipid contained 2.3 to 3.0% phosphorus, 2.5 to 3.0% carbohydrate, and a very small amount of nitrogen (0.2 to 0.3%). The phospholipid was of the phosphatidyl glycerol type. Electron micrographs of the washed membrane showed three layers. The outer and inner layers varied in thickness from 25 to 37 A and the middle layer from 20 to 25 A. The total thickness varied between 85 and 100 A. These preparations contained many vesicles which stained heavily with lead citrate. Some vesicles were also attached to the protoplast ghosts in the form of extrusions or intrusions, or both. Membrane preparations obtained by lysis of protoplasts in the absence of MgCl(2) were fragmented and contained less lipid (20 to 22%) and ribonucleic acid (0.3 to 0.5%) than preparations prepared with MgCl(2).

Micromorphology of Cryptococcus neoformans

Fine details of the internal and external morphology of Cryptococcus neoformans as seen in ultrathin sections are described and illustrated with electron micrographs. The capsule characteristic of this species contained microfibrils (30 to 40 A in diameter) that appeared to radiate from the cell wall and to coil and intertwine in various directions. These thin, uniformly structured, electron-dense filaments are believed to represent complex polysaccharide molecules. The internal morphology of C. neoformans was in many ways similar to that of yeasts studied by other authors. The cell was uninucleate with a single nucleolus. The nuclear envelope, a pair of unit membranes interrupted by pores, was typical of that found in eucaryotic organisms. Smooth endoplasmic reticulum, mitochondria, vacuoles, storage granules, and ribosomes were consistent features of the cytoplasm. In addition, C. neoformans presented membranous organelles derived from the plasma membrane and comparable to bacterial mesosomes and mitochondria of an annulate type.

Fine structure of Listeria monocytogenes in relation to protoplast formation

Among the eight strains of Listeria monocytogenes tested for lysozyme sensitivity, two were resistant to lysozyme but became sensitive after lipase pretreatment. Among the other six, one was very sensitive to lipase and another one was extremely susceptible to lysozyme. Stable protoplasts were formed from the lysozyme-resistant strain (42) by lipase and lysozyme treatment, which completely digested the cell wall. The cell wall (uranyl acetate-lead stained) was of a thick triple-layered profile, with the intermediate layer of low density. Lipase treatment for a short time (60 min) did not cause any alteration in structure, but prolonged treatment (180 min) caused extensive digestion of the plasma membrane and the cell wall, liberating cytoplasmic material. When the cells were treated with either lipase or lysozyme, a small number of protoplasts were extruded through the partly digested or weakened transverse cell wall, leaving an almost intact cell wall ghost. There were small vesicular structures in the interspace between cell wall and plasma membrane. Mesosomes of varied organization were prominent in electron micrographs, both in sections and in negatively stained preparations. These were largely everted during protoplasting in the form of tubules and as small peripheral buds; a few small vesicles also remained as intrusive structures, some of which were very unusual because they appeared to be enclosed by the inner layer of plasma membrane alone. Lysis of the protoplasts by dilution of the sucrose, while maintaining a constant ionic environment, liberated many small vesicular structures and fibrillar nuclear material.

Ultrastructure of the membrane system in Lactobacillus plantarum

Electron microscopic study of Lactobacillus plantarum revealed mesosomes in different stages of maturation and structural relation with other cell organelles. Small, immature mesosomes were bounded by a prominent electron-dense layer with another extremely faint layer on the outside. This corresponds to the appearance of the cytoplasmic membrane. Large mature mesosomes were surrounded by a triple-layered unit membrane having electron-opaque layers of approximately equal density, suggesting that the composition of the boundary membrane alters during development of this structure. Three-dimensional observations derived from serial sections indicated that mesosomes always maintain a connection between the cytoplasmic membrane and the comparable layers of their boundary. The cytoplasmic membrane also consisted of a triple-layered unit membrane, the innermost layer of which was less electron-opaque and was usually hidden by the relatively dense background of the cytoplasm. The innermost layer of the cytoplasmic membrane was most clearly seen in plasmolyzed cells. Only mature mesosomes made distinct contacts with, or were partially immersed in, the nucleoplasm. The boundary of such mesosomes frequently seemed to be discontinuous, suggesting that the mesosome interior was in direct contact with the nucleoplasm. Mesosomes involved in cross-wall formation at a division plane increased in size and passed through a sequence of positions which led ultimately to an association with the nucleoplasms of the daughter cells. The inner surface of the cell wall was lined by a thin, electron-dense layer whose composition and function are unknown. Under the cultural conditions used, this organism regularly contained a polyphosphate granule.

Fine structure of sporulation in Bacillus cereus grown in a chemically defined medium

Ellar, D. J. (Syracuse University, Syracuse, N.Y.), and D. G. Lundgren. Fine structure of sporulation in Bacillus cereus grown in a chemically defined medium. J. Bacteriol. 92:1748-1764. 1966.-A study was made of the fine structure of sporulating cells of Bacillus cereus grown in a chemically defined medium. The developmental stages of sporulation occurred in a fairly synchronous manner and were complete by 14 hr. This time period was shortened when spore wall peptide components were added to the medium, but the addition had no effect upon fine structure except to thicken the cell wall. Sporulation could be separated into six morphological stages which generally agreed with those published for other sporulating bacteria. The initiation of the spore (forespore) septum takes the form of an inward folding of the cytoplasmic membrane toward the pole of the cell. The inward folding forms a characteristic Y-shaped membrane structure enclosing an area within which vesicles are found. These vesicles comprise the perisporal mesosome of the cell. The membranes on opposite sides of the cell progress toward the cell center where they fuse to form the double unit membrane of the spore septum. As the proliferation of the spore septum continues, the vesicular areas move towards the pole. The end result is a double forespore membrane which completely encloses a part of the vegetative cell's chromatin. Sporal mesosomes, as well as membrane vesicles, are involved in the proliferation of the forespore. Vesicles are generally bounded by a single unit membrane, whereas in the sporal mesosomes several unit membranes are arranged concentrically. The latter become associated with the segregation of a portion of the nuclear material into the forespore region of the cell.

Beta-hemolytic gram-negative hemophilic bacillus isolated from the genitalia of rabbits

Muraschi, Thelma F. (Division of Laboratories and Research, New York State Department of Health, Albany), John K. Miller, Victor N. Tompkins, and Dorothy M. Bolles. beta-Hemolytic gram-negative hemophilic bacillus isolated from the genitalia of rabbits. J. Bacteriol. 91:356-366. 1966.-An unidentified beta-hemolytic gram-negative hemophilic bacillus was encountered during experiments for recovery of Listeria monocytogenes from rabbits after vaginal instillation. Cultural, morphological, biochemical, and serological properties of the isolate were studied, and comparisons were made with members of the genus Haemophilus and L. monocytogenes. Identity of the microorganism was not established.