Gerald J Domingue MD
“Demystifying Pleomorphic Forms in Persistence and Expression of Disease: Are They Bacteria, and Is Peptidoglycan the Solution?”
Review by Jon Trister MD
The interaction of microbes within the host can lead to the enhancement or depression of their individual properties. Clinical expression of their presence in the host depends on the genetic vulnerability of the host, the particular environmental stresses, and the number and location of such consortia. The clinician who faces this tangled scenario must quantitate and define the dynamic that has led to the patient’s illness.
Many bacteria-like elements can be visualized at the ultra-structural level, but cannot be grown in culture. Nucleic acid analyses in vitro can approximate the locations of these bacteria on the phylogenetic tree. Unfortunately, none of these sophisticated laboratory procedures are consistently successful in identifying pleomorphic organisms persisting in tissues, nor are they guides to optimal therapy.
Pleomorphic, cryptic organisms, whether intra- or extracellular, are ubiquitous. A first step would be to demonstrate their presence in tissue samples when laboratories report culture negative findings in patients suspected of having a bacterial infection; or to attempt to grow them in culture. Quantifying and identifying the cells most parasitized are impractical routine clinical approaches. Koch’s postulates cannot be fulfilled, because it is impossible to precisely duplicate all the variables that are involved in disease expression.
Any patient with a history of recurrent infections and persistent disability is sending signals that this phenomenon is occurring. The autoimmune disorders, in which no organisms can be identified by routine techniques, are suspect in this regard. The selection of antimicrobial agents for patients with cryptic infections can be quite frustrating. Even if an organism grows in vitro, it may not represent the primary pathogen. In addition, drug susceptibility testing fails to reveal the action of the agent on the infecting organism’s toxicity and capacity to adhere to cell membranes in vivo.
Although physicians are discouraged from the indiscriminate use of antimicrobial agents without strong cultural, immunologic, or molecular evidence that the therapy is appropriate and that the severity of the illness justifies the risk of side effects, it is, nevertheless, a common clinical practice and undoubtedly contributes to the development of pleomorphic, persisting bacterial forms and mutants in vivo.
Survival of a species requires that a reasonable identity be maintained. Over time, mechanisms to maintain “self” have evolved. Many such relationships have been so successful that both host and invading organism benefit. Such a process, which transiently reduces immune competence, can occur episodically in healthy subjects in association with various stresses .
As one ages, there tends to be an insidious accumulation of intracellular microbial forms. Such quiescent organisms tend to be activated to a more toxic form when homeostatic disturbances threaten their cellular loci. The numbers and locations of cells involved with one or more types of organisms determine the clinical reaction. It can be very difficult to decide whether a new illness is due to a new organism or to an interaction with one or more pleomorphic cryptic organisms. These interactions can be as complex as the well-known increase in toxicity of Corynebacterium diphtheriae when this bacterium is infected by bacteriophage. The distinction between phage genes and bacterial genes is blurred with respect to both function and reality. It is conceivable that much of the heredity of bacteria is of viral origin, because many unknown defective proviruses may exist in nature; on the other hand, phages may be fragments of bacterial DNA that have acquired the capacity for independent reproduction. Indeed, with a history of mutual interaction of viruses and bacteria over the course of evolution, the endeavor of sharply distinguishing their genes must be meaningless. These philosophic concepts are implicit in any discussion of the role of dormant, persistent, difficult-to-culture, and impossible-to-culture bacteria in disease (Domingue and Woody, 1997).
From the evidence available in the literature, it seems that mycoplasmas are a diverse group of wall-less prokaryotes derived from various bacteria. It has been convincingly demonstrated by immunologic methodology that acholeplasmas are descended from streptococci, specifically from groups N and D. It therefore seems logical to conclude from molecular and immunologic data that mycoplasmas are not a true phylogenetic class and that they are not descended from one single common ancestor. A teleologic approach to the evolutionary relationship between mycoplasmas and cell wall-defective/deficient bacteria should consider the survival advantage of an organism with a cell wall in a hostile primordial environment. Only after the appearance of higher life forms was there a protective niche for mutant microorganisms (Domingue and Woody, 1997).
If we are to extend these findings to clinical relevance, it is tempting to speculate that in vivo genetic events may lead to development of bacteria with aberrant cell wall morphology and physiology and may involve complex interactions among a variety of bacteria and host cells. Such interactions might lead to persistence of a dormant bacterial phase in patients with infectious diseases. This may be a continual biologic process in all living hosts, with the host environment serving as the determinant for evolution, persistence, and survival of morphologically altered microbes. Previously described, newly published, provocative, molecular microbiological data lend credence to the hypothesis and corroborate the multiplicity of pleomorphic forms that develop during reproduction of L-forms in vitro. Recent studies on modifications of gene expression and modes of division for stressed bacteria are highly relevant to the hypothesis. It is proposed that in vivo persistence of these bacterial elements escape immune surveillance partially, completely, or may integrate with host cell organelles to create bacteria-host-cell-antigen complexes which could provoke immunopathologic consequences. To speculate further, bacterial persisters in a scenario of molecular mimicry might possess peptide sequence similarities with self peptides sufficient to result in cross-activation of autoreactive T or B cells by pleomorphic form derived peptides.
Might there also be an analogous situation to that of H. pylori growth in the human stomach: Do persistent atypical bacterial forms produce enzymes which neutralize hostile host factors creating a more hospitable tissue environment; and are there microbial factors antagonistic to white blood cells preventing their migration to the infectious site or scene of pleomorphic form persistence? Furthermore, there may be an exchange of genetic material between the persisting prokaryote nucleoid (oncogenic plasmids or unknown nuclear interactions) and host eukaryotic chromosome creating cellular alterations adequate to initiate neoplastic growths.
Scientists skilled in disciplines such as cellular adhesion, transposition of genetic elements, and microbial reassembly as mechanisms for the genesis of unusual organisms should be able to design, execute, and interpret experimentation that will confirm or refute, unambiguously, the proposed hypothesis. If pleomorphic forms in tissues are confirmed as bacteria by sensitive and specific methodology, the clinical and research implications are unlimited; and have the potential for clarifying the mysterious and poorly understood host-pathogen interactions in persistent infections and expression of innumerable idiopathic diseases.