Microbiology Of Chlamydia

Almen L. Barron

• Publisher: CRC Press
• Year: 2019
• Print ISBN: 9780367227074
• eBook ISBN: 9781000013016
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

First Published in 1988, this book offers a full, comprehensive guide into microbiology of Chlamydia and its relationship with our bodies. Carefully compiled and filled with a vast repertoire of notes, pictures, and references this book serves as a useful reference for Students of Medicine, and other practitioners in their respective fields.

Chlamydia as Organisms

Chapter 1

CHARACTERISTICS OF CHLAMYDIAE

James W. Moulder

TABLE OF CONTENTS

I.	Introduction
II.	Properties of the Genus Chlamydia A. Definition of the Genus Chlamydia B. Differentiation of the Two Species of Chlamydia C. Differentiation of the Three Biovars of C. trachomatis
III.	Chlamydial Phylogeny A. Some Matters of Phylogenetic Importance B. A New Family Tree of Chlamydia
IV.	Chlamydial Nomenclature and Taxonomy A. Importance of a Rational Nomenclature and Taxonomy B. Future of Chlamydial Nomenclature
V.	Summary

References

I. INTRODUCTION

The chlamydiae have been assigned to the order Chlamydiales, which is comprised of one family, Chlamydiaceae, with a single genus, Chlamydia, and two species, C. trachomatis and C. psittaci.^1–3 The description of the genus Chlamydia in Bergey’s Manual of Systematic Bacteriology follows this classification,⁴ and it provides a generally satisfactory characterization of the chlamydiae infecting humans, other mammals, and birds. However, the present state of chlamydial taxonomy is unsatisfactory in at least two respects. First, it takes no account of the many reports of chlamydia-like organisms living intracellularly in invertebrate hosts, and second, being based solely on phenotypic similarities and differences, its relation to the evolutionary history of chlamydiae is uncertain.

This chapter will describe the definitive properties of the genus Chlamydia, how its species and biovars are distinguished one from the other, and how recent advances in chlamydial biology and bacterial phylogeny allow construction of a plausible evolutionary history of chlamydiae. Finally, it will consider ways in which the familiar C. trachomatis and C. psittaci and the as yet poorly characterized chlamydia-like inhabitants of invertebrates may all be accommodated within the order Chlamydiales, perhaps by the creation of new families and genera.

II. PROPERTIES OF THE GENUS CHLAMYDIA

A. Definition of the Genus Chlamydia

The genus Chlamydia is defined by the properties listed in Table 1. Presently, only strains of C. psittaci and C. trachomatis are known to fit this definition. With one exception, the chlamydia-like organisms of invertebrates have not been propagated in the laboratory. So, apart from morphology, their characteristics are largely unknown.

1. Obligate Intracellular Habitat

Although no chlamydia has so far been observed to grow extracellularly, either in nature or in the laboratory, serious and sustained efforts to achieve host-free multiplication have not been made. With our rapidly increasing understanding of chlamydial biology, such efforts may soon become worthwhile.

2. Developmental Cycle

Like many other intracellular parasites,⁵ chlamydiae have evolved morphologically distinct infectious and reproductive forms.^6–8 Chlamydial elementary bodies never divide. Their role is to carry the infection from one cell (or one host) to another, where they reorganize into reticulate bodies which multiply by binary fission in membrane-bound intracytoplasmic vacuoles or inclusions. Reticulate bodies do not infect new host cells. Instead, they reorganize into new generations of elementary bodies to complete the developmental cycle. The most critical structural difference between elementary bodies and reticulate bodies appears to be the extent to which their outer membrane proteins are complexed by disulfide cross-linking. These proteins are extensively cross-linked in elementary bodies but not in reticulate bodies.^9–15 There is a strong temptation to ascribe the many biological differences (Table 2) between the two chlamydial cell types to this structural difference, but direct evidence for a cause and effect relationship is largely lacking.

3. Gram Negative Envelope Without Peptidoglycan

Envelopes of both elementary bodies and reticulate bodies resemble those of host-indpendent Gram-negative bacteria in that they are made up of an inner cytoplasmic membrane and an outer membrane,^7,8,17,18 are disrupted by polymyxin B and ethylenediaminetetra-acetate,^19,20 and contain an outer membrane protein that accounts for half of the total protein of that membrane.^10,18,21,22 Chlamydial cell envelopes differ from those of typical Gram-negative bacteria in that they have no peptidoglycan. Electron micrographs show that there is no peptidoglycan layer between the inner and outer membranes,¹⁸ and chemical analysis reveals no muramic acid or any other amino sugar that might have replaced it in the peptidoglycan subunit.^23,24

Table 1
CHARACTERS THAT DEFINE THE GENUS CHLAMYDIA

Obligate intracellular habitat.

Developmental cycle with morphologically distinct infectious and reproductive forms.

Gram negative envelope without peptidoglycan.

Genus-specific lipopolysaccharide.

Patches of hexagonally arrayed cylindrical projections.

Utilization of host ATP for synthesis of chlamydial protein.

Small genome.

Table 2
BIOLOGICAL DIFFERENCES BETWEEN ELEMENTARY BODIES AND RETICULATE BODIES

Property	Elementary body	Reticulate body
Infectivity	Yes	No
Multiplication	No	Yes
Inhibition of phagosome-lysosome fusion	Yes	No
Toxic for mice	Yes	No
Toxic for macrophages	Yes	No
ATP transport	No	Yes
Protein synthesis	No	Yes

Modified from Moulder, J. W., ASM News, 50, 353, 1984. With permission.

Absence of peptidoglycan could mean, as has been suggested for the peptidoglycan-less budding bacteria,²⁵ that the chlamydiae branched off the main eubacterial tree before peptidoglycan was invented. However, it is more likely that chlamydiae have evolved from ancestors with peptidoglycan because they appear to have retained vestiges of a former peptidoglycan-containing state. Chlamydiae have penicillin-binding proteins similar in location, size, and affinity for the antibiotic to those of host-independent Gram-negative bacteria.²³ In low concentration, penicillin inhibits the growth and division of reticulate bodies and prevents their reorganization into elementary bodies.²⁶ Growth and multiplication of both C. trachomatis and C. psittaci are also blocked by D-cycloserine,^27,28 another inhibitor of peptidoglycan synthesis,²⁹ although C. trachomatis strains are usually much more susceptible. Penicillin inhibits the transpeptidation reaction responsible for the closing of the peptide cross-links in peptidoglycan,^29,30 whereas D-cycloserine inhibits both the formation of D-alanine from L-alanine and the synthesis of D-alanyl-D-alanine.²⁹ Sensitivity of chlamydial multiplication to these two antibiotics implies the presence of a D-alanyl-D-alanine sequence somewhere in the chlamydial cell. For this reason, it has been suggested that chlamydial envelopes contain D-alanyl-D-alanine peptides that are cross-linked to structures other than peptidoglycan.²⁴ The chlamydial susceptibility to inhibitors of peptidoglycan synthesis in the absence of peptidoglycan is without known parallel. The peptidoglycan-less budding bacteria are, for example, relatively resistant to both penicillin and D-cycloserine.²⁵

4. Genus-Specific Lipopolysaccharide

All isolates of C. trachomatis and C. psittaci so far examined contain a lipid-soluble complement-fixing antigen that is present at all times in the developmental cycle.³¹ This genus-specific antigen (formerly called the group antigen) strongly resembles the lipopolysaccharides (LPSs) of host-independent Gram negative bacteria in its location in the outer membrane of the chlamydial cell envelope,³² in its chemical structure,^33–35 and in its biological activity.^36,37 There is also strong immunologic cross-reaction between the chlamydial LPS and the innermost core of the LPS from Salmonella mutants in which that structure is exposed.^38–40 Monoclonal antibodies against chlamydial LPS reveal at least three antigenic domains, two of which are shared with the LPSs of some free-living Gram-negative o...

Table of contents

• Cover
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• CHLAMYDIA AS ORGANISMS
• CHLAMYDIA AS PATHOGENS
• Index