Morphology and reproductive processes of the L forms of bacteria. II. Comparative study of L forms and Mycoplasma with the electron microscope

Genetic requirements for cell division in a genomically minimal cell

Genomically minimal cells, such as JCVI-syn3.0, offer a platform to clarify genes underlying core physiological processes. Although this minimal cell includes genes essential for population growth, the physiology of its single cells remained uncharacterized. To investigate striking morphological variation in JCVI-syn3.0 cells, we present an approach to characterize cell propagation and determine genes affecting cell morphology. Microfluidic chemostats allowed observation of intrinsic cell dynamics that result in irregular morphologies. A genome with 19 genes not retained in JCVI-syn3.0 generated JCVI-syn3A, which presents morphology similar to that of JCVI-syn1.0. We further identified seven of these 19 genes, including two known cell division genes, ftsZ and sepF, a hydrolase of unknown substrate, and four genes that encode membrane-associated proteins of unknown function, which are required together to restore a phenotype similar to that of JCVI-syn1.0. This result emphasizes the polygenic nature of cell division and morphology in a genomically minimal cell.

Formation and characterization of non-growth states in Clostridium thermocellum: spores and L-forms

Background

Clostridium thermocellum is an anaerobic thermophilic bacterium that exhibits high levels of cellulose solublization and produces ethanol as an end product of its metabolism. Using cellulosic biomass as a feedstock for fuel production is an attractive prospect, however, growth arrest can negatively impact ethanol production by fermentative microorganisms such as C. thermocellum. Understanding conditions that lead to non-growth states in C. thermocellum can positively influence process design and culturing conditions in order to optimize ethanol production in an industrial setting.

Results

We report here that Clostridium thermocellum ATCC 27405 enters non-growth states in response to specific growth conditions. Non-growth states include the formation of spores and a L-form-like state in which the cells cease to grow or produce the normal end products of metabolism. Unlike other sporulating organisms, we did not observe sporulation of C. thermocellum in low carbon or nitrogen environments. However, sporulation did occur in response to transfers between soluble and insoluble substrates, resulting in approximately 7% mature spores. Exposure to oxygen caused a similar sporulation response. Starvation conditions during continuous culture did not result in spore formation, but caused the majority of cells to transition to a L-form state. Both spores and L-forms were determined to be viable. Spores exhibited enhanced survival in response to high temperature and prolonged storage compared to L-forms and vegetative cells. However, L-forms exhibited faster recovery compared to both spores and stationary phase cells when cultured in rich media.

Conclusions

Both spores and L-forms cease to produce ethanol, but provide other advantages for C. thermocellum including enhanced survival for spores and faster recovery for L-forms. Understanding the conditions that give rise to these two different non-growth states, and the implications that each has for enabling or enhancing C. thermocellum survival may promote the efficient cultivation of this organism and aid in its development as an industrial microorganism.

Transitional forms between the three domains of life and evolutionary implications

The question as to the origin and relationship between the three domains of life is lodged in a phylogenetic impasse. The dominant paradigm is to see the three domains as separated. However, the recently characterized bacterial species have suggested continuity between the three domains. Here, we review the evidence in support of this hypothesis and evaluate the implications for and against the models of the origin of the three domains of life. The existence of intermediate steps between the three domains discards the need for fusion to explain eukaryogenesis and suggests that the last universal common ancestor was complex. We propose a scenario in which the ancestor of the current bacterial Planctomycetes, Verrucomicrobiae and Chlamydiae superphylum was related to the last archaeal and eukaryotic common ancestor, thus providing a way out of the phylogenetic impasse.

Permanent alterations of the L-forms of proteus and salmonella under various conditions

L-forms obtained from three strains of Proteus and from one strain of Salmonella have been kept for 15 to 20 years by weekly or monthly transfers on agar plates containing penicillin. The morphology and growth requirements of these strains have changed. They now grow abundantly on the surface of agar and in broth. The cultures consist of large bodies, small granules, and transitional forms. These organisms are more resistant to distortion and stain more deeply than organisms of the usual L-forms. In broth and to a lesser extent on agar, branching filaments develop, on the ends of which both the large, round organisms and small organisms are produced. The filaments are a transitional stage in the development of the cultures. Usual bacillary forms were not present in the culture and did not appear in successive transfers in the absence of penicillin. Bacilli reappeared on exposure of the L cultures to the influence of a spore-bearing bacillus. A similar transformation of L-forms has also been observed developing within a short time in recently isolated A and B type L cultures of one Proteus strain during the process of reversion to the bacterial form. The altered cultures are fixed in a stage of transition between the B type L-form and the regular bacteria.

Bacterial persistence and expression of disease

A considerable body of experimental and clinical evidence supports the concept that difficult-to-culture and dormant bacteria are involved in latency of infection and that these persistent bacteria may be pathogenic. This review includes details on the diverse forms and functions of individual bacteria and attempts to make this information relevant to the care of patients. A series of experimental studies involving host-bacterium interactions illustrates the probability that most bacteria exposed to a deleterious host environment can assume a form quite different from that of a free-living bacterium. A hypothesis is offered for a kind of reproductive cycle of morphologically aberrant bacteria as a means to relate their diverse tissue forms to each other. Data on the basic biology of persistent bacteria are correlated with expression of disease and particularly the mechanisms of both latency and chronicity that typify certain infections. For example, in certain streptococcal and nocardial infections, it has been clearly established that wall-defective forms can be induced in a suitable host. These organisms can survive and persist in a latent state within the host, and they can cause pathologic responses compatible with disease. A series of cases illustrating idiopathic conditions in which cryptic bacteria have been implicated in the expression of disease is presented. These conditions include nephritis, rheumatic fever, aphthous stomatitis, idiopathic hematuria, Crohn's disease, and mycobacterial infections. By utilizing PCR, previously nonculturable bacilli have been identified in patients with Whipple's disease and bacillary angiomatosis. Koch's postulates may have to be redefined in terms of molecular data when dormant and nonculturable bacteria are implicated as causative agents of mysterious diseases.

Formation and cultivation of Borrelia burgdorferi spheroplast-L-form variants

As clinical persistence of Borrelia burgdorferi in patients with active Lyme borreliosis occurs despite obviously adequate antibiotic therapy, in vitro investigations of morphological variants and atypical forms of B. burgdorferi were undertaken. In an attempt to learn more about the variation of B. burgdorferi and the role of atypical forms in Lyme borreliosis, borreliae isolated from antibiotically treated and untreated patients with the clinical diagnosis of definite and probable Lyme borreliosis and from patient specimens contaminated with bacteria were investigated. Furthermore, the degeneration of the isolates during exposure to penicillin G in vitro was analysed. Morphological analysis by darkfield microscopy and scanning electron microscopy revealed diverse alterations. Persisters isolated from a great number of patients (60-80%) after treatment with antibiotics had an atypical form. The morphological alterations in culture with penicillin G developed gradually and increased with duration of incubation. Pleomorphism, the presence of elongated forms and spherical structures, the inability of cells to replicate, the long period of adaptation to growth in MKP-medium and the mycoplasma-like colonies after growth in solid medium (PMR agar) suggest that B. burgdorferi produce spheroplast-L-form variants. With regard to the polyphasic course of Lyme borreliosis, these forms without cell walls can be a possible reason why Borrelia survive in the organism for a long time (probably with all beta-lactam antibiotics) [corrected] and the cell-wall-dependent antibody titers disappear and emerge after reversion.

Morphologic study of Staphylococcus aureus L-form, reverting, and intermediate colonies in situ

Staphylococcus aureus strains of bovine origin were induced to L-form by exposure to 100 U of penicillin in brain heart infusion broth supplemented with 5% NaCl, 5% sucrose, and 10% horse serum. L-forms were cultured on similarly supplemented brain heart infusion agar containing no antibiotic. Light and electron microscopic examination of plastic-embedded L-form colonies revealed a variety of morphologic types. The primary site of growth appeared to be the core area below the agar surface, consisting mainly of pleomorphic budding forms. At the surface, these forms gave rise to large spherules with a gradation from smaller to larger spherules toward the periphery of the colony. Some colonies progressed to reverting forms with the growth of bacterial cells containing cell wall. In addition to L-forms, intermediate colony forms were observed that lacked typical L-form morphology and progressed rapidly to the parent cell form on subculture to bovine blood agar. Description of these forms will be used in the search for similar morphologic types in vivo during antibiotic treatment of chronic S. aureus bovine mastitis.

Characterization of a stable spheroplast type L-form of Proteus mirabilis D 52 as cell envelope mutant. II. Electronmicroscopic investigations

The ultrastructural analysis of a stable spheroplast type L-form of Proteus mirabilis D 52 revealed characteristic alterations in the organization of the cell envelope including defective changes in the cell envelope structure as for instance the loss of a coherent murein layer, the loss of some components in the outer cell wall layer, the formation of small membraneous vesicles at the tips of loose extensions of the cell wall, a decrease in associations and bindings between wall and membrane, an extension of the periplasmic space, an increase in membrane defects, as well as a disturbed cell division causing unusual modes of multiplication and the formation of various intracellular structures like membrane complexes, characteristic sheet-like membraneous bodies, typical inclusion bodies, and defective phage structures, which all could not be observed in normal rod-shaped cells. The results of these investigations and of those given in a previous paper (Gumpert and Taubeneck 1975) show, that the stable spheroplast type L-form LD 52 B must be considered as a true cell envelope mutant in which the biosynthesis and structure of the cell envelope is altered genetically by one or several mutations whereas the main biochemical activities are the same like those of the parent bacterium. The profound alterations in the cell envelope system, however, lead to some changes in the whole cell organization, which apparently in turn cause disorders even in metabolic and biosynthetic processes not directly involved in the biosynthesis of the cell envelope.

Proposed reproductive cycle for a relatively stable L-phase variant of Streptococcus faecalis

Based on electron microscopic observations, the reproductive cycle of a relatively stable L-phase variant of Streptococcus faecalis was proposed. The small, dense, nonvesiculated L-form is envisioned as the central (core) element. It was seen to divide and bud rapidly. In addition, the dense forms appeared to be capable of growth and development within vesicles of mature mother forms. When these forms were released from the vesicles into the surrounding fluid medium, further growth occurred, resulting in the development of immature and ultimately mature mother forms. Under conditions unfavorable for L-form growth, these dense forms developed first into transitional forms and then into the bacterial form. These dense forms might therefore be considered as undifferentiated "stem cells' with the capacity to develop along several different routes, depending upon the stimulus received.

Morphology and ultrastructure of staphylococcal L colonies: light, scanning, and transmission electron microscopy

Scanning electron microscopy utilizing critical point drying was used in parallel with light and transmission electron microscopy to study L colonies produced by a stable L-phase variant of Staphylococcus aureus (AH24H).

Fine structure of protoplasts and L-forms of Clostridium botulinum types A and E

Spherical bodies, that are obtained by adding penicillin and lysozyme to Clostridium botulinum types E and A cultures which are growing in an osmotically stabilized medium, are shown to be protoplasts by electron microscopy. The L-forms of these two culture types have morphologically different inclusion bodies.

Growth of streptococcal protoplasts and L-colonies on membrane filters

Membrane filters (Millipore Corp.; pore sizes 1.2 to 0.22 mum) were placed on the surface of L-phase growth medium solidified with agar. The filter and the surrounding medium were inoculated with either protoplasts or stable broth-grown L-phase variants obtained from Streptococcus faecium strain F24. The L-phase inoculum gave rise to viable L-colonies on the filters and on the medium, whereas protoplasts gave colony formation only on the medium. However, when the Millipore filters were covered by a layer of solid L-phase medium, 75 mum or greater in depth, before inoculation with protoplasts, colony formation resulted but with atypical morphology. In contrast, inoculation of protoplasts on Nuclepore and Sartorius membrane filters did give rise to L-colonies on the surface and underneath the filters after 2 days of incubation at 37 C. Submicroscopic, viable L-phase elements produced during colony formation were capable of passing through membrane filters with pore channels as small as 0.22 mum; these elements required transfer from underneath the filters to fresh agar medium in order to develop into L-phase colonies. Membrane filters were also placed on the surface of L-phase growth medium solidified with gelatin. Inoculation of the filters and surrounding medium with a lysozyme-prepared protoplast suspension gave rise to streptococci on the surface of the filters and on the medium. However, inoculation with the stable broth-grown L-phase variants gave rise to atypical colonies on the medium and only small patches of abortive growth on the filters.

Filterability of streptococcal L-forms

The filterability of the broth-grown stable L-form derived from Streptococcus faecium F24 was tested by filtration under the influence of varying amounts of applied pressure. A decrease in the pore size of the filter resulted in a corresponding decrease in viable count, but no major effect was noted due to the different pressures applied. Serial filtration of a deoxyribonuclease-treated L-form culture in the mid-logarithmic phase of growth resulted in recovery of viable L-forms from the 0.45-mum filtrate but not from the 0.22-mum filtrate. It is possible that disruption of the L-form bodies with release of small viable elements had occurred. Protoplasts, diluted in an osmotic stabilizer, were filtered similarly; L-forms could be grown from the filtrate passing through the filters of 0.45 mum or greater. Filtration of the parent streptococci gave rise to streptococcal colonies from the 1.2-mum filtrate only.