Actinomycetes

Actinomycetes are a group of metabolically dynamic microorganisms. They are better known for the production of a wide range of antibiotics, anticancer drugs, and industrial enzymes. Only a small number of species belonging to the order Actinomycetales are pathogenic. From the evolutionary perspective, actinomycetes form a bridge between the classic bacteria and fungi. The properties that mandate their classification with bacteria include prokaryotic cell structure and in vitro sensitivity to most antibacterial antibiotics. On the other hand, their tendency to form true branching with septate mycelium, external spores, and granulomatous tissue reaction in infected hosts bring them closer to fungi. The term “actinomycete” literally means “ray fungus.” Because of their fungus-like properties, actinomycetes have been traditionally studied by mycologists. By early 1970s, it became obvious that these are prokaryotic microorganisms, and most mycologists started avoiding them, but bacteriologists did not quite embrace them either. As a result the study of actinomycetes has fallen into a “no man’s land.”

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Spirochetes and Bacteria without a Cell Wall

Spirochetes and bacteria without a cell wall do not quite fit in with the classic con- cepts of bacteria that have been discussed so far. It is also a fact that there is no similarity between the members of the two groups; they are very different from each other. They are discussed here in one chapter only for the sake of brevity.

SPIROCHETES

Spirochetes are spiral, Gram-negative bacteria with a unique mode of motility that is quite different from those of other bacteria (they lack external flagella). All bacteria classified as spirochetes generally have a helical protoplasmic cylinder made of a thin layer of peptidoglycan and a multilayered outer membrane. Spirochetes differ considerably from each other with respect to habitats and physiological characteristics. Three genera are associated with serious diseases in humans. These are Treponema, Borrelia, and Leptospira.

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Miscellaneous Gram-Negative Bacteria

This chapter covers several Gram-negative but unrelated taxa. We did not mean to make this chapter a dumping ground, but they are all included in this chapter simply for the sake of brevity. The important taxa include the following:

• Brucella melitensis

• Bordetella pertussis

• Francisella spp.

• Pasteurella spp.

• Vibrio cholerae

• Campylobacter spp.

• Helicobacter spp.

• Legionella spp.

• Gardnerella vaginalis

• Chlamydia spp.

• Rickettsia rickettsii

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Gram-Negative Bacilli

Asporogenous Gram-negative bacilli of clinical importance can be divided into two major groups. Glucose-fermenting, oxidase-negative, and catalase-positive members constitute one group, called Enterobacteriaceae. Several members of this group are normally present in human intestines and others are causal agents of serious infec- tions. The second group, somewhat more heterogeneous, usually called nonfermentative Gram-negative bacilli, are glucose nonfermenters. They are widely distributed in nature and prefer aquatic habitats. However, several members of this group are frequently isolated from human sources and known to cause serious infections. A simple and practical scheme for the grouping of important pathogenic Gram-negative bacteria is depicted at the end of the previous chapter.

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Gram-Negative Cocci

For a general discussion on Gram-negative bacteria, readers are referred to the last part of this chapter. With reference to Gram-negative cocci, members of the follow- ing three genera are considered important pathogens:

• Neisseria spp.

 N. gonorrhoeae

 N. meningitidis

• Moraxella spp.

 M. catarrhalis

• Haemophilus (It is a coccobacillus and sometimes listed among the Gram- negative bacilli)

 H. influenzae

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Gram-Positive Bacteria with Rudimentary Filaments

Taxonomically, bacteria with rudimentary filaments rank between classic bacilli and the Actinomycetes, bacteria with branching filaments. They represent an important evolutionary link in microbiology. Among the bacteria with rudimentary filaments, Corynebacterium and Mycobacterium are two important pathogenic genera.

CORYNEBACTERIUM  DIPHTHERIAE

Members of the genus Corynebacterium are aerobic to facultative anaerobic, catalase positive, and do not produce spores. They are common constituents of resident microbiota on human skin and in the mouth and upper respiratory tract. More than 16 species are recognized. Of these, C. diphtheriae is the most important pathogenic species. Like several other causal agents of diseases, asymptomatic carriers of C. diphtheriae are not uncommon.

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Gram-Positive Bacilli

Important pathogens in this group of bacteria are restricted to three genera that include aerobes as well as anaerobes:

• Clostridium spp. (anaerobic)

• Bacillus spp. (aerobic)

• Listeria spp. (aerobic to facultative anaerobic)

Most Gram-positive rods grow well on blood agar. However, selective media are available in some cases, for example, cycloserine–cefoxitin–fructose–egg yolk agar (CCFA) for C. difficile.

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Gram-Positive Cocci

BACTERIAL TAXONOMY (AN OVERVIEW)

The classic approach to the classification of bacteria is based on size and shape, later aided by reaction to Gram stain. As advances in microscope-making were made and simple biochemical tests became available, gradually the system became more refined. The science of taxonomy matured with the availability of the electron microscope and advances in molecular biology. Muller may have been the first biologist to attempt to classify bacteria in late 18th century, but his attempts were limited by lack of understanding of bacteria and crudeness of microscopes. Cohn, in late 19th century, made further advances, but serious attempts to classify bacteria were first made only in 20th century. Bergey’s Manual of Determinative Bacteriol- ogy was first published in 1923 and it instantly became the foundation of and the most authoritative source for bacterial taxonomy. Aided by inputs from the American Society for Microbiology, international societies for bacterial taxonomy and nomen- clature, and the International Journal of Systemic Bacteriology, Bergey’s manual has been greatly refined and its scope enlarged. Its 9th edition was published in 1994. Another remarkable contribution to bacterial taxonomy was made by the publication of Bergey’s Manual of Systematic Bacteriology, a five-volume set that examines bacterial taxonomy in greater detail. Students interested in bacterial taxonomy are encouraged to consult these highly authoritative sources.

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Antiseptics and Disinfectants

The terms “antiseptic” and “disinfectant” are often confused and misused in micro- biology and medicine. Typically, “antiseptic” refers to an agent used to minimize, destroy, or remove microbial population on a living surface, such as the skin of a person who needs to be prepared for injection or a surgical procedure. A disinfectant, on the other hand, is a substance used to eliminate or minimize microbial presence on an inanimate surface, such as a work bench, glassware, or surgical instruments. It is noteworthy that both antiseptics and disinfectants can either be a microbicide or a microbistatic. In this chapter, we will treat the two entities together under the banner of control of microbial population. The microbial population in or on a surface, material, or product can be controlled, minimized, or eradicated either by physical means or by chemical means, some of which are summarized below.

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Antibiotics and Other Chemotherapeutic Agents

Technically, the antibiotic era began with the discovery of penicillin by Sir Alexan- der Fleming in 1929. However, its development could occur only during World War II. By that time, an energetic soil scientist, Dr. Selman Waksman, had established a school of soil microbiology in New Jersey’s Rutgers University. Focusing on soil- borne aerobic actinomycetes, his group started a systematic program that lead to the discovery of streptomycin, an antibiotic credited with saving lives of millions of tuberculosis patients all over the world. Since then, his group at Rutgers as well as his students in various educational and industrial research laboratories went on to discover thousands of antibiotics, which include almost all the powerful drugs, such as tetracycline, erythromycin, chloramphenicol, amphotericin B, and vancomycin. Traditionally, the term “antibiotics” has been used for the antimicrobial agents derived from microorganisms. Since a number of antibiotics currently in use are actually synthetic, the term antibiotics has become synonymous with antimicrobial agents used for the treatment of infectious diseases.

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Host-Microbe Interactions

The human body reacts in many different ways to microorganisms. These interac- tions can be summarized in the following categories:

RESIDENT MICROBIOTA

All surfaces of the human body, including the skin as well as the mucous membranes that surround the inner parts of the mouth, nostrils, genitals, and gastrointestinal tract, are inhabited by a distinct set of microbial communities, which are specifically adapted to the local physical and chemical environment. Such normal microorgan- isms, called resident microbiota, perform extremely important roles.

1. Resident microbiota engage all available binding sites on the host cell sur- faces, thus invaders have a diminished possibility of attaching to the host cell surfaces.

2. Many microbes secrete vitamins that are absorbed by the host and serve important nutritional needs.

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TRANSMISSION OF INFECTIOUS DISEASE (MODE OF DISSEMINATION)

Airborne (Inhalation of Bioaerosols) 

A bioaerosol contains bacteria in its center, surrounded by air and a small amount of liquid, generally saliva. Bioaerosols may be produced due to sneezing, coughing, or talking. Depending on the force of sneezing or coughing, the bioaerosol-borne microorganism can travel up to several meters in air. Almost all respiratory tract infections are airborne; some can also pass from person to person through the inhala- tion of bioaerosols. Some of the examples of airborne infections include tuberculo- sis, strep throat, diphtheria, pertussis, legionellosis, influenza, and chicken pox, and a wide range of mycotic diseases such as aspergillosis, zygomycosis, cryptococcosis, histoplasmosis, and coccidioidomycosis.

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MAJOR CATEGORIES OF PATHOGENIC MICROORGANISMS

Viruses

Believed to be a bridge between the living and the nonliving, viruses have either DNA or RNA, seldom both. Their genome is surrounded by a protein coat, called capsid. Certain viruses have an envelope, often derived from the host cell membrane during lysis and release. Multiple characteristics, including type of nucleic acid, single or double strands, and presence or absence of envelope, are taken into account in the classification of viruses.

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