INTRODUCTION TO MICROBIAL PHYSIOLOGY are sythes d at the membrane surface and 9 embone水指 eg sed o The proteins in the cytoplasmic membrane thickness brane while others functio The outer membrane of gran egativecel contansa relatively high These lipi ntaining componens on formed by the extemal hat is, it is respon protein receptor sites fo Once attached Gram-positive and gram-negative cell have som ewhat different strategies fo into the cell However.chemicals and nutrients must first trave the outer membrane of gram organisn the cytoplas nic membran Gram-negativ passage of fairly large moleculesint the periplasmic space.Subse ent transpor cross the r or cytoplasmic membrane is similar in both gram-positive and gram Capsules 0 bacteri d of and u acid in alternating 1,3 and 1,4 linkages: compose ccal poly
4 INTRODUCTION TO MICROBIAL PHYSIOLOGY present in gram-positive organisms) — are synthesized at the membrane surface and may extend through the peptidoglycan layer to the outer surface. The peptidoglycan layer of a gram-negative cell is generally a single monolayer. An outer membrane surrounding the gram-negative cell is composed of phospholipids, lipopolysaccharides, enzymes, and other proteins, including lipoproteins. The space between this outer membrane and the inner membrane is referred to as the periplasmic space. It may be traversed at several points by various enzymes and other proteins (Fig. 1-2). Membranes. The cytoplasmic membrane of both gram-positive and gram-negative cells is a lipid bilayer composed of phospholipids, glycolipids, and a variety of proteins. The proteins in the cytoplasmic membrane may extend through its entire thickness. Some of these proteins provide structural support to the membrane while others function in the transport of sugars, amino acids, and other metabolites. The outer membrane of gram-negative cells contains a relatively high content of lipopolysaccharides. These lipid-containing components represent one of the most important identifying features of gram-negative cells: the O antigens, which are formed by the external polysaccharide chains of the lipopolysaccharide. This lipidcontaining component also displays endotoxin activity — that is, it is responsible for the shock observed in severe infections caused by gram-negative organisms. Bacterial cell surfaces also contain specific carbohydrate or protein receptor sites for the attachment of bacteriophages, which are viruses that infect bacteria. Once attached to these receptor sites, the bacteriophage can initiate invasion of the cell. Gram-positive and gram-negative cells have somewhat different strategies for transporting materials across the membrane and into the cell. The cytoplasmic membrane of gram-positive organisms has immediate access to media components. However, chemicals and nutrients must first traverse the outer membrane of gramnegative organisms before encountering the cytoplasmic membrane. Gram-negative cells have pores formed by protein triplets in their outer membrane that will permit passage of fairly large molecules into the periplasmic space. Subsequent transport across the inner or cytoplasmic membrane is similar in both gram-positive and gramnegative cells. Capsules. Some bacterial cells produce a capsule or a slime layer (Fig. 1-4) of material external to the cell. Capsules are composed of either polysaccharides (highmolecular-weight polymers of carbohydrates) or polymers of amino acids called polypeptides (often formed from the D- rather than the L-isomer of an amino acid). The capsule of Streptococcus pneumoniae type III is composed of glucose and glucuronic acid in alternating β-1, 3- and β-1, 4- linkages: O O O O O O O O O OH OH OH OH HO OH CH2OH COOH CH2OH COOH OH HO This capsular polysaccharide, sometimes referred to as pneumococcal polysaccharide, is responsible for the virulence of the pneumococcus. Bacillus anthracis, the anthrax
CELL STRUCTURE 5 Fig.1-4.Capsules of Streptococcus pneumoniae s a virulenc contains a motor that turns the flagelum.which propels the orism throu the liguid environment .Many bacteria possess extemal that are shorter andmor structu termed pili (fromL in me aning"hai Generalized or common Riboso look.Ribosomes contain approximately 65%RNA and 35%protein (see Fig.1-1)
CELL STRUCTURE 5 Fig. 1-4. Capsules of Streptococcus pneumoniae. bacillus, produces a polypeptide capsule composed of D-glutamic acid subunits, which is a virulence factor for this organism. Organs of Locomotion. Many microorganisms are motile — that is, able to move from place to place in a concerted manner — especially in an aqueous environment. In the case of bacteria, this motility is accomplished by means of simple strands of protein (flagellin) woven into helical organelles called flagella. The bacterial flagellum is attached at the cell surface by means of a basal body (Fig. 1-5a). The basal body contains a motor that turns the flagellum, which propels the organism through the liquid environment. Pili or Fimbriae. Many bacteria possess external structures that are shorter and more rigid than flagella. These structures have been termed pili (from Latin meaning “hair”) or fimbriae (from Latin meaning “fringe”). These appendages also appear to arise from a basal body or granule located either within the cytoplasmic membrane or in the cytoplasm immediately beneath the membrane (Fig. 1-5b). Generalized or common pili play a role in cellular adhesion to surfaces or to host cells. Ribosomes. The cytoplasm of all cells has a fine granular appearance observed in many electron micrographs. Tiny particles called ribosomes are responsible for this look. Ribosomes contain approximately 65% RNA and 35% protein (see Fig. 1-1)
6 1 um The ribosom ed unit denotes the rate of sedimentation of a macromolecule in a centrifugal field and
6 INTRODUCTION TO MICROBIAL PHYSIOLOGY 1 µm 0.05 µm (a) (b) Fig. 1-5. Microbial appendages. (a) Flagella of Salmonella typhimurium. (b) Pili of Escherichia coli. (Source: Pili Image courtesy Indigo Instruments. Visit http://www.indigo.com.) Reprint permission is granted with this footer included. The ribosome orchestrates the polymerization of amino acids into proteins (i.e., protein synthesis). At higher magnification under the electron microscope the ribosome particles are spherical. In properly prepared specimens the ribosomes are observed as collections or chains held together on a single messenger RNA (mRNA) molecule and are referred to as polyribosomes or simply polysomes. The more or less spherical ribosome particle, when examined by sucrose gradient sedimentation, has been found to have a svedberg coefficient of 70S. (A svedberg unit denotes the rate of sedimentation of a macromolecule in a centrifugal field and
SYNTHESIS OF DNA.RNA.AND PROTEIN 7 another of 30.Only the complete particle functions in polypeptide synthesis By compar 80S mes obably io and mitochondrial genomes SYNTHESIS OF DNA,RNA,AND PROTEIN The chromosome of E.coli is a single,circular,double-stranded DNA molecule whose nuc sequence encodes all the information required for cell growth and emormOecpNAeeiqSredarpopaatnheecsrMtth6 replication involve ccurate duplication of chro osomal DNA and the formatio ded DNA is shown in oin he e he Leading Strand DNA polymerase RNA primer Okazaki Fragment、 Lagging Strand Fig.1-6.Simplified depiction of DNA replication
SYNTHESIS OF DNA, RNA, AND PROTEIN 7 is related to the molecular size of that macromolecule.) The prokaryotic ribosome may be separated into two lower-molecular-weight components: one of 50S and another of 30S. Only the complete 70S particle functions in polypeptide synthesis. By comparison, the ribosomes of eukaryotic cells are associated with the endoplasmic reticulum, are larger (80S), and are composed of 40S and 60S subunits. The function of both 70S and 80S ribosomes in protein synthesis is identical. Curiously, eukaryotic mitochondria characteristically display 70S ribosomes — not the 80s particles that you would expect — because mitochondria probably evolved from endosymbiotic prokaryotic cells, a hypothesis supported by extensive analyses comparing bacterial and mitochondrial genomes. SYNTHESIS OF DNA, RNA, AND PROTEIN The chromosome of E. coli is a single, circular, double-stranded DNA molecule whose nucleotide sequence encodes all the information required for cell growth and structure. The major molecular events required for propagating the species start with the chromosome and include DNA replication, transcription, and translation. In bacteria, replication involves the accurate duplication of chromosomal DNA and the formation of two daughter cells by binary fission. In binary fission the cell grows until a certain mass-to-DNA ratio is achieved, at which point new DNA is synthesized and a centrally located cross-wall is constructed that will ultimately separate the two daughter cells. A simplified view of DNA replication in E. coli is shown in the diagram in Figure 1-6. The double-stranded DNA molecule unwinds from a specific starting point (origin). The new DNA is synthesized opposite each strand. The enzyme involved in 5′ 5′ 3′ Unwinding Enzyme RNA primer Okazaki Fragment Leading Strand Lagging Strand 5′ 5′ DNA polymerase RNA Primer Removed 5′ Gap filled, Nick sealed Fig. 1-6. Simplified depiction of DNA replication
证身 省料 密 柔 Fig.1-7.Segregation of the bacterial chromosome. es unti both replicatio
8 INTRODUCTION TO MICROBIAL PHYSIOLOGY Fig. 1-7. Segregation of the bacterial chromosome. replication (DNA polymerase) uses a parent strand as a template, placing adenine residues opposite thymine, and cytosine residues opposite guanine. New DNA is synthesized in both directions from the origin and continues until both replication forks meet at the terminus 180◦ from the origin. At this point, cell division proceeds with cross-wall formation occurring between the two newly synthesized chromosomes