Microbiology Lecture 2 - Bacteria PDF

PROKARYOTE CELL STRUCTURE AND FUNCTIONS » Prokaryotic cells are cells that lack a true, membrane-enclosed nucleus. E.g. bacteria, blue-green algae Size, Shape and Arrangement » Most bacteria have one or two shapes. » Cocci (Coccus) are spherical cells. Can exist as individual cells or in charact...

PROKARYOTE CELL STRUCTURE AND FUNCTIONS » Prokaryotic cells are cells that lack a true, membrane-enclosed nucleus. E.g. bacteria, blue-green algae Size, Shape and Arrangement » Most bacteria have one or two shapes. » Cocci (Coccus) are spherical cells. Can exist as individual cells or in characteristic arrangement useful in bacterial identification. Saturday, October 29, 2022 1 » Diplococci (Diplococcus) when cocci divide and remain together to form pairs. » Long chains of cocci result when cells adhere after repeated divisions in one plane, often seen in the genera Streptococcus, Enterococcus and Lactococcus. 2 » Staphylococcus divides in random planes to generate irregular grapelike clumps. » Members of the genus Micrococcus often divide in two planes to form square groups of four cells called Tetrads. 3 » Genus Sarcina, Cocci divide in three planes producing cubical packets of eight cells. Saturday, October 29, 2022 4 Try this Saturday, October 29, 2022 5 » Other common types of bacterial shape is rod shape often called Bacilli (Bacillus). Shape of rod’s end may be flat, rounded, cigar-shaped or bifurcated. May occur single or remain together after division to form pairs or chains. » Coccobacilli Saturday, October 29, 2022 6  Bacilli megaterium—rods in chains Saturday, October 29, 2022 7 » Few rod-shaped bacteria, Vibrios are curved to form distinctive commas or incomplete spirals. Saturday, October 29, 2022 8 »Many bacteria are shaped like long rods twisted into spirals or helices. They are spirilla if rigid and spirochetes when flexible. »A few bacteria are flat. »Some bacteria are variable in shape and lack a single characteristic form. These are called pleomorphic. 9  R. rubrum—spiral-shaped spirilla Actinomyces—a filamentous bacterium Leptospira interrogans—a spirochete Saturday, October 29, 2022 10 The diversity of bacteria. (a) Pseudomonas aeruginosa, a rod-shaped, flagellated bacterium (bacillus). Pseudomonas includes the bacteria that cause many of the most serious plant diseases. (b) Streptococcus. The spherical individual bacteria (cocci) adhere in chains in the members of this genus (X34,000). (c) Spirillum volutans, one of the spirilla. This large bacterium, which occurs in stagnant fresh water, has a tuft of flagella at each end. Saturday, October 29, 2022 11 Saturday, October 29, 2022 12  Haloquadratum walsbyi, a square archaeon Saturday, October 29, 2022 13 Saturday, October 29, 2022 14 » Bacteria vary in size as much as in shape. » The smallest (e.g. members of genus Mycoplasma) are about 0.3 µm in diameter. » Nanobacteria and utramicrobacteria appear to range from around 0.2 µm to less than 0.05 µm in diameter. » E. coli, a Bacillus is 1.1 to 1.5 µm wide by 2 to 6 µm long. Saturday, October 29, 2022 17 »Some spirochates occasionally reach 500 µm in length and cyanobacterium, Osillatoria is about 7 µm in diameter. »Epulopiscium fishelsoni grows as large as 600 by 80 µm. Saturday, October 29, 2022 18  PROKARYOTE CELL ORGANIZATION » Bounded by a chemically complex cell wall. » Separated from it by a periplasmic space lies the plasma membrane. » Genetic material is the nucleoid, not separated from cytoplasm by membranes. » Ribosomes and inclusion bodies scattered about in the cytoplasmic matrix. » Flagella for locomotion. » Pili for attachment to surfaces and mating. » Some are surrounded by a capsule or slime layer external to the cell wall. Saturday, October 29, 2022 19 Saturday, October 29, 2022 Prokaryote cell organization 20  Morphology of a prokaryotic cell. Saturday, October 29, 2022 21 Plasma membrane Selectively permeable membrane, mechanical boundary of cell, nutrient and waste transport, location of many metabolic processes (respiration, photosynthesis), detection of environmental cues for chemotaxis. Gas Vacuole Buoyancy for floating in aquatic environments Ribosomes Protein synthesis Inclusion bodies Storage of carbon, phosphate and other substances Nucleoid Localization of genetic material Periplasmic space Contains hydrolytic enzymes and binding proteins for nutrient processing and uptake Cell wall Shape and protection from lysis in dilute solutions Capsules and slime layers Resistance to phagocytosis, adherence to surfaces Fimbriae and pili Attachment to surfaces, bacterial mating Flagella Movement Endospore Survival under harsh conditions 22  BACTERIAL CELL WALL »The cell wall is the layer, usually fairly rigid, that lies just outside the plasma membrane. It is one of the most important prokaryotic structures for several reasons; It helps determine the shape of the cell. It helps protect the cell from osmotic lysis. It can protect the cell from toxic substances. It can contribute to pathogenicity. Saturday, October 29, 2022 23 »Bacteria could be divided into two major groups based on their response to the Gram-stain procedure. »Gram-positive bacteria stain purple, whereas gram-negative bacteria stain pink or red by the technique. Saturday, October 29, 2022 24 GRAM-POSITIVE BACTERIA CELL WALL »The gram-positive cell wall consists of a single, 20 to 80 nm thick homogeneous layer of peptidoglycan (murein) lying outside the plasma membrane. Saturday, October 29, 2022 25 GRAM-NEGATIVE BACTERIA CELL WALL »In contrast, the gram-negative cell wall has a 2 to 7 nm peptidoglycan layer covered by a 7 to 8 nm thick outer membrane. Saturday, October 29, 2022 26  M, peptidoglycan or murein layer, OM, outer membrane; PM, plasma membrane; P, periplasmic space; W, gram- positive peptidoglycan wall. Saturday, October 29, 2022 27 The Gram stain. The peptidoglycan layer encasing gram-positive bacteria traps crystal violet dye, so the bacteria appear purple in a Gram-stained smear (named after Hans Christian Gram, who developed the technique). Because gram- negative bacteria have much less peptidoglycan (located between the plasma membrane and an outer membrane), they do 28 not retain the crystal violet dye and so exhibit the red background stain (usually a safranin dye). Peptidoglycan » Peptidoglycan is an enormous, mesh like polymer composed of many identical subunits. The polymer contains two sugar derivatives, N -acetylglucosamine and N -acetylmuramic acid (the lactyl ether of N -acetylglucosamine), and several different amino acids. » The backbone of this polymer is composed of alternating N -acetylglucosamine and N -acetylmuramic acid residues. A peptide chain of four alternating D - and L - amino acids is connected to the carboxyl group of N -acetylmuramic acid. Many bacteria replace meso -diaminopimelic acid with another diaminoacid, usually L – lysine. Saturday, October 29, 2022 29 EXTERNAL COMPONENTS OF THE BACTERIAL CELL WALL Saturday, October 29, 2022 30 Capsules and Slime layers » Some prokaryotes have a layer of material lying outside the cell wall. This layer has different names depending on its characteristics. » When the layer is well organized and not easily washed off, it is called a capsule. It is called a slime layer when it is a zone of diffuse, unorganized material that is removed easily. » When the layer consists of a network of polysaccharides extending from the surface of the cell, it is referred to as the glycocalyx, a term that can encompass both capsules and slime layers because they usually are composed of polysaccharides. Saturday, October 29, 2022 31 »Some slime layers and capsules are constructed of other materials. E.g. Bacillus anthracis has a proteinaceous capsule composed of poly- D - glutamic acid. »Capsules are clearly visible in the light microscope when negative stains or special capsule stains are employed they also can be studied with the electron microscope. Saturday, October 29, 2022 32  Bacterial Capsules. (a) Klebsiella pneumoniae with its capsule stained for observation in the light microscope (×1,500). (b) Bacteroides glycocalyx (gly), TEM (×71,250). Saturday, October 29, 2022 33 Bacterial Glycocalyx. Bacteria connected to each other and to the intestinal wall by their glycocalyxes, the extensive networks of fibers extending from the cells (×17,500). 34 Functions of Capsules/Slime layers/Glycocalyx »They help pathogenic bacteria resist phagocytosis by host phagocytes. E.g. Streptococcus pneumoniae. When it lacks a capsule, it is destroyed easily and does not cause disease. »Capsules contain a great deal of water and can protect against desiccation. Saturday, October 29, 2022 35 »The glycocalyx also aids in attachment to solid surfaces, including tissue surfaces in plant and animal hosts. Saturday, October 29, 2022 36 Fimbriae »Many prokaryotes have short, fine, hair like appendages. These are usually called fimbriae. »They are slender tubes composed of helically arranged protein subunits and are about 3 to 10 nm in diameter and up to several micrometres long. »A cell may be covered with up to 1,000 fimbriae, but they are only visible in an electron microscope due to their small size. Saturday, October 29, 2022 37  The numerous shorter fimbriae are evident in this electron micrograph of the bacterium Proteus vulgaris (×39,000). Saturday, October 29, 2022 38 Functions of Fimbriae »Some types of fimbriae attach bacteria to solid surfaces such as rocks in streams and host tissues, and some are involved in motility. Saturday, October 29, 2022 39 Pili »Pili often are larger than fimbriae (around 9 to 10 nm in diameter). »They are genetically determined by conjugative plasmids and are required for conjugation. Saturday, October 29, 2022 40 Saturday, October 29, 2022 41 Flagella » Most motile prokaryotes move by use of flagella (flagellum), threadlike locomotor appendages extending outward from the plasma membrane and cell wall. » Bacterial flagella are slender, rigid structures, about 20 nm across and up to 20 μm long. Flagella are so thin they cannot be observed directly with a bright-field microscope but must be stained with special techniques designed to increase their thickness. » The detailed structure of a flagellum can only be seen in the electron microscope. Saturday, October 29, 2022 42  The long flagella are evident in this electron micrograph of the bacterium Proteus vulgaris (×39,000). Saturday, October 29, 2022 43 Types of Flagella » Bacterial species often differ distinctively in their patterns of flagella distribution, and these patterns are useful in identifying bacteria. » Monotrichous bacteria (trichous means hair) have one flagellum; if it is located at an end, it is said to be a polar flagellum. » Amphitrichous bacteria (amphi means on both sides) have a single flagellum at each pole. » lophotrichous bacteria (lopho means tuft) have a cluster of flagella at one or both ends. » Flagella are spread evenly over the whole surface of peritrichous (peri means around) bacteria. 44 Flagellar Distribution. Examples of various patterns of flagellation as seen in the light microscope. (a) Monotrichous polar (Pseudomonas). (b) Lophotrichous (Spirillum). (c) Peritrichous (Proteus vulgaris, ×600). 45  EUKARYOTE CELL ORGANIZATION Saturday, October 29, 2022 46 » Eukaryotic cells have membrane bound nuclei, and membranes play a prominent part in the structure of many other organelles. » Organelles are intracellular structures that perform specific functions in cells analogous to the functions of organs in the body. » The partitioning of the eukaryotic cell interior by membranes makes possible the placement of different biochemical and physiological functions in separate compartments so that they can more easily take place simultaneously under independent control and proper coordination. Saturday, October 29, 2022 47 » Large membrane surfaces make possible greater respiratory and photosynthetic activity because these processes are located exclusively in membranes. » The intracytoplasmic membrane complex also serves as a transport system to move materials between different cell locations. » Thus, abundant membrane systems probably are necessary in eukaryotic cells because of their large volume and the need for adequate regulation, metabolic activity, and transport. Saturday, October 29, 2022 48 Saturday, October 29, 2022 49 Organization levels of the body  MICROBIAL GROWTH Saturday, October 29, 2022 51 »Growth may be defined as an increase in cellular constituents. It leads to a rise in cell number when microorganisms reproduce. »Growth also results when cells simply become longer or larger. »If the microorganism is coenocytic – i.e. a multinucleate organism in which chromosomal replication is not accompanied by cell division growth results in an increase in cell size but not cell number. Saturday, October 29, 2022 52 »The cell cycle is the complete sequence of events extending from the formation of a new cell through to the next division. »The understanding the cell cycle has practical importance for clinicians. Why??? »Cell cycle e.g. binary fission. Saturday, October 29, 2022 53 Binary fission »The cell elongates. »Replicates its chromosome and separates the newly formed DNA molecules so there is one chromosome in each half of the cell. »Finally, a septum (cross wall) is formed at midcell, dividing the parent cell into two progeny cells, each having its own chromosome and a complement of other cellular constituents. Saturday, October 29, 2022 54 Saturday, October 29, 2022 55  MICROBIAL GROWTH Saturday, October 29, 2022 56 Saturday, October 29, 2022 57 Saturday, October 29, 2022 58 Saturday, October 29, 2022 59  PATHOGENESIS OF BACTERIAL INFECTIONS/DISEASES Saturday, October 29, 2022 60 ☺The steps for infections by pathogenic bacteria usually include the following: 1. Maintain a reservoir. 2. Initial transport to and entry into the host. 3. Adhere to, colonize, or invade host cells or tissues. 4. Evade host defense mechanisms. 5. Multiply (grow) or complete its life cycle on or in the host’s cells. 6. Damage the host. 7. Leave the host and return to the reservoir or enter a new host. Saturday, October 29, 2022 61 1. Maintain a reservoir ☺All bacterial pathogens must have at least one reservoir. ☺The source or reservoir of the pathogen is part of the infectious disease cycle. ☺E.g. are Saturday, October 29, 2022 62 2. Initial transport to and entry into the host ☺Direct contact - from reservoir to host or host to another host (inhalation, contact, coughing, sneezing, spitting, body contact). ☺Indirect contact – E.g. fomites Saturday, October 29, 2022 63 3a. Attachment ☺The first line body defenses are very effective in removing organism, thus a pathogen must adhere to host cells as a necessary first step in the establishment of infection. ☺They bind to host cells using adhesins. (Adherance factors). ☺Adhesins can be surface structures such as capsules/slime layer/glycocalyx or various cell wall proteins. 64 » Surface receptors on human cells to which bacterial adhesins attach are typically glycoproteins or glycolipids (protein molecules that have various sugars attached), the adhesins bind to the sugar component of the receptors. » Binding of adhesins to a surface receptor is highly specific, dictating the type of cells to which the bacteria can attach. E.g. adhesins of most strains of E. coli allows them to adhere to intestinal cells. » Other pathogenic strains of E. coli have other additional adhesins that allows them to attach to other tissues. E.g. P pili mediate specific attachment to cells that line the bladder to cause UTI etc. [refer to table]. Saturday, October 29, 2022 65 Saturday, October 29, 2022 66 ☺Strains that cause watery diarrhea produce a type of pili that allows the bacteria to attach specifically to cells of the small intestine. Saturday, October 29, 2022 67 3b. Colonization ☺To colonize a site that is populated by normal flora, the new pathogen must compete successfully with established organisms for space and nutrients and overcome their toxic products such as fatty acids and bacteriocins. Saturday, October 29, 2022 68 ☺Iron is required for the growth of most pathogenic bacteria. However, the concentration of free iron in the human body is fairly low because most of the iron is tightly bound to iron-transport proteins, such as lactoferrin, transferrin, and ferritin, as well as haemoglobin. ☺To obtain iron, some pathogens secrete proteins called siderophores. Siderophores are released into the medium, where they take the iron away from iron-transport proteins by binding the iron even more tightly Saturday, October 29, 2022 69 ☺Once the iron-siderophore complex is formed, it is taken up by siderophore receptors on the bacterial surface. ☺Then the iron is brought into the bacterium. In some cases, the iron is released from the complex to enter the bacterium; in other cases, the iron enters as part of the complex. ☺As an alternative to iron acquisition by siderophores, some pathogens have receptors that bind directly to iron-transport proteins and haemoglobin. Then these are taken into the bacterium directly along with the iron. ☺Also, it is possible that some bacteria produce toxins when iron levels are low. The toxins kill host cells, releasing their iron and thereby making it available to the bacteria. Saturday, October 29, 2022 70 Saturday, October 29, 2022 71 Delivery of effector molecules to host cells ☺Once they have colonized a surface, some bacteria are able to deliver certain molecules directly to host cells inducing changes in those cells. E.g. gastrointestinal pathogens. ☺In some cases, the compounds induce changes that damage recipient host cell structures such as loss of microvilli. ☺Gram negative bacteria use type III secretion systems to deliver proteins to eukaryotic cells. They resemble short flagella and function as microscopic hypodermic needles. Saturday, October 29, 2022 72 Type III Secretion System (a) X-ray fiber diffraction resolves the injectisome as a helical structure. (b) Scanning tunneling electron microscopy reveals the injectisome tip, indicating how it may lock into the translocator pore on the target cell. 73 4. Evade host defense mechanisms Evading the complement system ☺To evade the activity of complement, some bacteria have capsules that prevent complement activation. Some gram-negative bacteria can lengthen the O chains in their lipopolysaccharide to prevent complement activation. ☺ Others such as Neisseria gonorrhoeae generate serum resistance. These bacteria have modified lipooligosaccharides on their surface that interfere with proper formation of the membrane attack complex during the complement cascade. Saturday, October 29, 2022 74 »The virulent forms of Neisseria gonorrhoeae that exhibit serum resistance are able to spread throughout the body of the host and cause systemic disease, whereas those Neisseria gonorrhoeae that lack serum resistance remain localized in the genital tract. Saturday, October 29, 2022 75 Resisting phagocytosis ☺Phagocytosis involves - chemotaxis, recognition and attachment, engulfment, fusion of phagosome with lysosome=digestion of microbe. ☺Some bacteria such as Streptococcus pneumoniae, Neisseria meningitis, and Haemophilus influenzae can produce a slippery mucoid capsule that prevents the phagocytosis. ☺They do this by binding to host cell complement regulatory proteins that inactivates C3b that has bound to the surface. Saturday, October 29, 2022 76 Saturday, October 29, 2022 77 Saturday, October 29, 2022 Slippery mucoid capsule of Acinetobacter calcoaceticus 78 »Other bacteria evade phagocytosis by producing specialized surface proteins such as the M protein on Streptococcus pyogenes. »Like capsules, these proteins interfere with adherence between a phagocytic cell and the bacteria. M proteins binds to complement regulatory proteins that inactivates C3b, interfering with complement activation and the subsequent formation of more C3b. Saturday, October 29, 2022 79  M proteins on S. pyogenes Saturday, October 29, 2022 80 ☺Some pathogens avoid phagocytosis by avoiding macrophages and neutrophils altogether. E.g. Staphylococcus produces leukocidins that destroy phagocytes before phagocytosis can occur. ☺C5a peptidase - this enzyme degrades the complement protein C5a, a chemoattractant that recruits phagocytic cells to an area where complement has been activated. E.g. Streptococcus pyogenes ☺Membrane damaging proteins – these kill phagocytes and other cells by forming pores in their membranes. E.g. Streptococcus pyogenes makes a membrane damaging toxin called steptolysin. Saturday, October 29, 2022 81  Staphylococcous aureus elaborates Leukocidin AB to mediate escape from within human neutrophils Saturday, October 29, 2022 82 Survival within the phagocyte ☺Escape from the phagosome - One method of evasion is to escape from the phagosome before it merges with the lysosome, as seen with Listeria monocytogenes, Shigella, and Rickettsia. ☺These bacteria use actin-based motility to move within host cells and spread between them. Upon lysing the phagosome, they gain access to the host cell cytoplasm, where they activate the assembly of an actin tail using host cell actin and other cytoskeletal proteins. Saturday, October 29, 2022 83 Saturday, October 29, 2022 84 ☺The actin tails propel the bacteria through the cytoplasm of the infected cell to its surface, where they push out against the plasma membrane and form protrusions. ☺The bacteria once again enter phagosomes and escape into the cytoplasm. In this way, the infection spreads to adjacent cells. The lysosomes never have a chance to merge with the phagosomes. Saturday, October 29, 2022 85  Formation of Actin Tails by Intracellular Bacterial Pathogens. (a) Transmission electron micrograph of Lysteria monocytogenes in a host macrophage. The bacterium has polymerized host actin into a long tail that it uses for intracellular propulsion and to move from one host cell to another. (b) Burkholderia pseudomallei (stained red) also forms actin tails as shown in this confocal micrograph. Note that the actin tails enable the bacterial cells to be propelled out of the host cell. Saturday, October 29, 2022 86 ☺Preventing (overcoming) phagosome lysosome fusion - Another approach is to resist the toxic products released into the phagolysosome after fusion occurs. ☺A good example of a bacterium that is resistant to the lysosomal enzymes is Mycobacterium tuberculosis probably at least partly because of its waxy external layer. ☺Still other bacteria prevent fusion of phagosomes with lysosomes E.g. Chlamydia trachomatis, Salmonella typhi Saturday, October 29, 2022 87 Avoiding antibodies ☺IgA protease – this enzyme cleaves IgA, the class of antibody found in mucus and other secretions. Neisseria gonorrhea and a variety of other pathogens produce IgA protease. ☺Antigenic variation – some pathogens routinely alter the structure of their surface antigens. This allows them to stay ahead of antibody production by altering the very molecules the antibodies would recognize. E.g. Neisseria gonorrhea makes genetic variations in its pili through a process called phase variation, resulting in altered pili protein sequence and expression. Saturday, October 29, 2022 88 ☺Some bacteria produce proteins (such as staphylococcal protein A and protein G of Streptococcus pyogenes) that interfere with antibody- mediated opsonization by binding to the Fc portion of immunoglobulins. ☺Mimick host tissue cells - some bacteria E.g. Streptococcus pyogenes produce capsules that are not antigenic because they resemble host tissue components. These capsules are composed of hyaluronic acid, a polysaccharide found in tissues. Saturday, October 29, 2022 89 5. Multiplication ☺For a bacterial pathogen to be successful in growth and reproduction, it must find an appropriate environment (e.g. nutrients, pH, temperature) within the host. 7 ☺Those areas of the host’s body that provide the most favourable conditions will harbour the pathogen and allow it to grow and multiply to produce an infection and subsequently the disease. Saturday, October 29, 2022 90 » Some bacteria can actively grow and multiply in the blood. The presence of viable bacteria in the bloodstream is called bacteraemia. »The infectious disease process caused by bacteria or their toxins in the blood is termed septicaemia. Saturday, October 29, 2022 91 6. Damage the host 1. Attacking the extracellular matrix and basement membranes of integuments and intestinal linings. 2. Degrading carbohydrate-protein complexes between cells or on the cell surface. 3. Disrupting the cell structure. Saturday, October 29, 2022 92 »The bacterial pathogen may continue disseminating throughout the body of the host. One way the pathogen accomplishes this is by producing specific structures or enzymes that promote spreading (virulent factors) [refer to table]. »Bacteria may also enter the small terminal lymphatic capillaries that surround epithelial cells. These capillaries merge into large lymphatic vessels that eventually drain into the circulatory system. Once the circulatory system is reached, the bacteria have access to all organs and systems of the host. Saturday, October 29, 2022 93 ☺Bacterial invasiveness varies greatly among pathogens. E.g. Clostridium tetani (cause of tetanus) produces a variety of virulence factors e.g. toxins and proteolytic enzymes but is considered non-invasive because it does not spread from one tissue to another. ☺Bacillus anthracis (cause of anthrax) and Yersinia pestis (cause of plague) also produce substantial virulence factors e.g. capsules and toxins but are highly invasive. ☺Members of the genus Streptococcus span the spectrum of virulence factors and invasiveness. Saturday, October 29, 2022 94 7. Leaving the host ☺The last determinant of a successful bacterial pathogen is its ability to leave the host and enter either a new reservoir host or a reservoir. ☺Unless a successful escape occurs, the disease cycle will be interrupted and the microorganism will not be perpetuated. 95 😎Three common portals of exit are the;  respiratory tract via coughing, sneezing, spitting, talking  gastrointestinal tract via saliva, vomit, feces  genitourinary tract via secretions from the vagina or penis. 😎Arthropods and syringes provide a portal of exit for microbes in blood. Saturday, October 29, 2022 96  TOXIGENICITY ☺Two distinct categories of disease can be recognized based on the role of the bacteria in the disease-causing process:  Infections.  Intoxications. ☺Host damage in an infection results primarily from invasiveness. Intoxications are diseases that result from a specific toxin e.g. botulinum toxin 97 ☺A toxin is a substance, such as a metabolic product of the organism, that alters the normal metabolism of host cells with deleterious effects on the host. Toxemia = toxins that have entered the blood of the host. ☺Some toxins are so potent that even if the bacteria that produced them are eliminated the disease conditions persist. ☺Toxins produced by bacteria can be divided into two main categories: exotoxins and endotoxins. Saturday, October 29, 2022 98  Exotoxins & Endotoxins Saturday, October 29, 2022 99 Saturday, October 29, 2022 100  IMPORTANCE OF BACTERIA ☺Autotrophic bacteria either photosynthetic or chemoautotrophic make major contributions to the carbon balance in terrestrial, freshwater, and marine habitats. ☺Heterotrophic bacteria play a key role in world ecology by breaking down organic compounds. ☺One of the most important roles of bacteria in the global ecosystem relates to the fact that only a few genera of bacteria and no other organisms have the ability to fix atmospheric nitrogen and thus make it available for use by other organisms. Saturday, October 29, 2022 101 » Bacteria are used in the production of acetic acid and vinegar, various amino acids and enzymes, and especially in the fermentation of lactose into lactic acid, which coagulates milk proteins and is used in the production of almost all cheeses, yogurt, and similar products. Saturday, October 29, 2022 102 ☺In the production of bread and other foods, the addition of certain strains of bacteria can lead to the enrichment of the final product with respect to its mix of amino acids, a key factor in its nutritive value. Many products traditionally manufactured using yeasts, such as ethanol, can also be made using bacteria. ☺Many of the most widely used antibiotics, including streptomycin, aureomycin, erythromycin, and chloromycetin, are derived from bacteria. ☺Bacteria can also play a part in removing environmental pollutants. Saturday, October 29, 2022 103