Practical Medical Microbiology for Clinicians

Frank E. Berkowitz, Robert C. Jerris

• Éditeur: Wiley-Blackwell
• Année: 2015
• ISBN:
  9781119067115
• Édition: 1
• Sujet: Sciences biologiques
• Sous-sujet: Microbiologie

Infectious diseases constitute a major portion of illnesses worldwide, and microbiology is a main pillar of clinical infectious disease practice. Knowledge of viruses, bacteria, fungi, and parasites is integral to practice in clinical infectious disease.

Practical Medical Microbiology is an invaluable reference for medical microbiology instructors. Drs. Berkowitz and Jerris are experienced teachers in the fields of infectious diseases and microbiology respectively, and provide expert insight into microorganisms that affect patients, how organisms are related to each other, and how they are isolated and identified in the microbiology laboratory. The text also is designed to provide clinicians the knowledge they need to facilitate communication with the microbiologist in their laboratory.

The text takes a systematic approach to medical microbiology, describing taxonomy of human pathogens and consideration of organisms within specific taxonomic groups. The text tackles main clinical infections caused by different organisms, and supplements these descriptions with clinical case studies, in order to demonstrate the effects of various organisms.

Practical Medical Microbiology is an invaluable resource for students, teachers, and researchers studying clinical microbiology, medical microbiology, infectious diseases, and virology.

SECTION III
Bacteriology

CHAPTER 9
Bacteriology

Structure of bacteria

Bacteria are prokaryotes, i.e. they do not have a nuclear membrane. They have a cell wall, a cell membrane, a chromosome, and ribosomes. In addition, some bacteria have flagella (singular flagellum), which enables these bacteria to move, and some have fimbriae (singular fimbria), also called pili (singular pilus), which enable the bacteria to adhere to epithelial surfaces (see virulence factors, later in this chapter), and to pass genetic material to other bacteria (see conjugation, later in this chapter). The basic structure of a bacterial cell is shown in Fig. 9.1.

Fig. 9.1 Structure of a bacterial cell.

Bacteria have a single chromosome which is arranged in a circle, and which is supercoiled, which reduces its volume. DNA replication occurs in both directions. In addition, they may have pieces of extrachromosomal DNA, called plasmids. These may contain genes for virulence factors or antibiotic resistance. The ribosomes, where proteins are synthesized, consist of 30S and 50S subunits. These are targets of antibiotics that inhibit protein synthesis.

There are three main shapes of bacteria: round – coccus (cocci); rod-shaped – bacillus/rod (bacilli/rods); spiral – spirochete(s).

Cell wall (Fig. 9.2)

Fig. 9.2 Basic structure of Gram-positive and Gram-negative cell walls.

This is a rigid structure, composed of cross-linked carbohydrates, that gives the organism shape and prevents osmotic injury. Enzymes necessary for its synthesis are present in the cell membrane. These are called penicillin-binding proteins and are targets of β-lactam antibiotics. In Gram-positive bacteria, the cell wall consists of about 40 layers of peptidoglycan, while in Gram-negative bacteria, it consists of 3–4 layers of peptidoglycan, outside which there is a complex lipid membrane called the outer membrane. A component of this membrane is endotoxin. Channels called porins allow macromolecules to pass through the outer membrane.

Genetic changes

Bacteria can change their genetic composition over a short period of time by the following mechanisms.

• Mutation: because bacteria have short generation times, mutations during replication can accumulate rapidly. Some of these may give the organisms a survival advantage.
• Acquisition of genetic material, by various mechanisms.
  • Conjugation: this is by the transfer of plasmids from a donor bacterial cell to a recipient cell. The plasmid DNA can replicate as does that of the chromosome. A plasmid that can be integrated into the chromosome is called an episome.
  • Transformation: this is the acquisition of DNA from the environment. This phenomenon was first demonstrated when live non-encapsulated pneumococci were mixed in a medium with dead encapsulated pneumococci. The live pneumococci acquired the ability to make capsules, indicating their ability to take up genetic material from the medium. This observation contributed to the discovery of the structure of DNA.
  • Transduction: this is the process by which a bacteriophage (a virus infecting a bacterium) introduces DNA into the bacterial cell. This DNA can be incorporated into the bacterial DNA (in which case it is called lysogenic), or it may remain in the cytosol, in which case it will replicate and cause lysis of the bacterial cell (lytic). An example of genetic material conferred by a bacteriophage is the gene for diphtheria toxin production in Corynebacterium diphtheria (see Chapter 12).

Pieces of genetic material that can move from a plasmid to a chromosome or to another plasmid are called transposons. Genes that can be carried in these genetic elements contain information that can convey antibiotic resistance and virulence. Groups of virulence genes constitute a pathogenicity island.

Bacteria...