Bacteria have contributed significantly to the rise in infectious diase. Bacteria as a class include a huge number of organisms, generally classified by biochemical characteristics (i.e., Gram stain characteristics) or morphological characteristics (e.g., bacilli cocci).
Bacteria are classified taxonomically as to genera and species. Many bacteria are harmless to humans while others cause disease only opportunistically. A subset of bacteria are highly pathogenic.
Differences between Bacterial and Eukaryotics
Bacterial are single cell prokaryotes, which have the following differences with the eukaryotic cell:
(1) Gram-Negative outer membrane: is similar in composition to most membrane (e.g., contains phospholipids) except that it contains specialized types of polysacharides and proteins. The uppermost layer of the OM is different because some of the phospholipid molecules are replaced with lipopolysaccharide (LPS). The lipid portion of LPS is referred to as an endoxtoxin because it sitimulates fever and shock reactions in gram-negative infections such as meningitis and typhoid fever.
The innermost layer of the OM is a phospholipid layer anchored by means of lipoproteins to the peptidoglycan layer below.
The OM serves allows only relatively small molecules to penetrate via special channels formed by porin proteins.
The OM contributes an extra barrier in gram-negative bacteria that makes them resistant to some antibiotics and antimicrobial chemicals such as dyes and disinfectants. For this reason, they are generally more difficult to inhibit or kill than gram-positive bacteria. One exception is for alcohol based compounds which can dissolve the lipids in the OM and damage the cell. This is why alcohol swabs are often used to cleanse the skin prior to certian medical procedures, such as venipuncture.
(2) surrounded by a cell wall: which is external to the cytoplasmic membrane, helps determine the shape of a bacterium and provides support to keep a bacterium from bursting becasue of changes in osmotic pressure.
The cell wall of most bacterial is relatively rigid because it is composed of peptidoglycan which interfers with phagocytosis, is mitogenic (stimulates mitosis of lymphocytes) and has pyrogenic activity (induces fever).
The B-lactam antibiotics (penicillins and cephalosporins) inhibit peptidoglycan synthesis. Cell envelope architecture differs between gram +/- bacteria. Humans cotain lysozyme, an enzyme contained in tears and saliva, which breaks the bonds in the glycan chains and cuases the cell wall to break down.
Plants, fungi and most protists also have cell walls but with a chemical structure different form that of peptidoglycan.
The bulk of gram-positive cell walls is a thick, homogeneous sheath of peptidoglycan ranging from 20-80 nm in thickness. It also contains tightly bound acidic polysaccharides including teichoic acid and lipoteichoic acid. Teichoic acid is a polymer of ribitol or glycerol (alcohols) and phosphate that is embedded in the peptidoglycan matrix. Lipoteichoic acid is similar in structure but is attached to the lipids in the cytoplasmic membrane. These molecuels appear to function in cell wall maintenance and enlargement during cell division and they contribute to the acidic charge on the cell surface.
The gram-negative cell wall is a single, think sheet of peptidoglycan.
Several bacterial groups have a different cell wall structure than gram positive or gram negative bacteria and some have no cell wall. Although these exceptions can stain positive or negative in the Gram stain, there are differences. For example, the cells of Mycobacterium and Nocardia contain peptidoglycan and stain gram-positive, but the bulk of their cell wall is composed of unique types of lipids. One of these is a very long chain fatty acid called mycolic acid which contributes to the pathogenicity of this group. The thick, waxy nature given to the cell wall by these lipids also means that they are highly resistant to certain chemicals and dyes. Such resistance is the basis for the acid fast stain used to diagnose tuberculosis and leprosy.
Mycoplasmas such as Mycoplasma pneumoniae which adheres to epithelial cells in the lung and causes an atypical from of pneeuonia called “walking pneuonia” are bacteria that naturally lack a cell wall. Although other bacteria require an intact cell wall to prevent the bursting of the cell, the mycoplasma cytoplasmic membrane is stabilized by serols and is resistant to lysis. These very tiny, pleomorphic cells are very small, ranging form 0.1-0.5 um.
(3) the cytoplasm is surrounded by a cytoplasmic membrane. (this is true with both gram + and -).
The CM is a lipid bilayer with proteins embedded. Some environmental bacteria, including phtosynthesizers and ammonia oxidizers contain dense stacks of internal membranes. In many cases, they derive from the cytoplasmic membrane and are studded with enzymes or photosynthetic pigments. The inner membranes allow for a higher concentration of these enzymes and pigments and also create compartmentalization that allows for higher energy production.
Because bacteria have none of the typical eukaryotic organelles, the CM provides a site for functions such as energy reactions, nutrient processing and synthesis.
(4) Various structures like flagella (long helical) and pili (straight and smaller) may protrude from the cell. Some bacteria contain appenddages that provide attachment points or channels (fimbriae, pili and nanotubes/nanowires).
Fimbria are small, bristle-like fibers sprouting off the surface of many bacteria. Most of them contain protein and have an inherent tendency to stick to each other and to surfaces. They are partially responsible for teh mutual clinging of cells that leads to biofilms. Some pathogens can colonize and infect host tissues because of a tight attachment between their fimbriae and epithelial cells. For example, Neisseria gonnorhoeae colonizes the genitourinary tract and Escherichia coli colonizes the intestine in this way. Mutant forms of these pathogens that lack fimbriae are unable to cause infections.
A pilus is a long, rigid tubular structure made of a special protein, pilin. Conjugation pili are utilized in a mating process between cells during conjugation which involves partial transfer of DNA from one cell to another. A conjugation pilus from the donor cell unites with a recipient cell, thereby providing a cytoplasmic connection for making the transfer. Conjugation using a pilus takes place only between compatible gram-negative cells. Although conjugation occurs in gram-positive bacteria, it does not involve a conjugation pilus.
The Type IV Secretion System (T4SS) is a complex molecular machine that often uses a pilus (specifically, a Type IV pilus) as a crucial component to extend, attach to recipient cells, and facilitate the transport of DNA or proteins, like a syringe and needle system. The T4SS itself includes the pilus and the channel, while the pilus is the movable, hair-like appendage that forms the initial connection and conduit.
The presence of pili is a characteristic most common among pathogenic bacteria of the mucosal surfaces like neisseria gonorrhoeae which are able to adhere to genital tract mucosal surfaces.
Nanotubes or nanowires are very thin, long, tubular extensions of the cytoplasmic membrane that bacteria use as channels either to trasnfer amino acids among one another or to harvest energy by shuttlign electrons from an electron rich surface in the environment. Bacteria can use this to generate energy in the absence of oxygen. For example, many bacteria can generate their energy in teh absence of oxygen by sharing electorns with substances in the environment containing iron, ushc as iron rich rocks. They use long anowires to do this, transferring electrons up and down the tubular extensions of the membrane.
(5) smaller than eukaryotic cells. Bacteria are about 1 micron whereas eukaryotes are typically greater than 5 microns. The genomes of bacteria are also smaller (2-5 million base pairs which encode about 4k genes)
(6) some bacterial (particularly gram -) contain additional DNA molecules called “plasmids” which replicate independently of the chromosomes and which often code for virulence factors.
Plasmids exist as seaprate double stranded circles of DNA, although at times they can become integrated into the chromosome. In certain cases, they may be duplicated and passed to related nearby bacteria via conjugation and transductionand occasionally by transformation.
(7) Bacteria reproduce by a process called binary fission rather than mitosis. Since there is only one DNA molecule, a spindle apparatus and other components of the mitotic cycle are unnecessary.
(8) there are no other organelles in their cytosol except ribosomes and inclusion bodies.
Bacteria lack a true membrane bound nucleus. Instead, the prokaryotic DNA molecule is found within the cytoplasm of the cell in a discrete area known as the nucleotid. Because there is no physical separation between transcription and translation as with eukaryotes, the two processes are “coupled” (occur simultaneously).
Bacterial ribosomes consist of a small subunit with an S value of 30 and a large subunit with an S value of 50. Overall, the density is 70S (it is not siply an additve property). Eukaryotic robosomes on the other hand are designated 80S.
Bacteria manurfacture structures called inclusion bodies to respond to their environment. They can store nutrients in inclusion bodies to respond to periods of low food availability. They can also pack gas into inclusion vesciles to provide buoyancy in an aquatic environment. They can even store crysals of iron oxide with magnetic properties, making themselves magnetotactic to orient themselves in polar and gravitational fields to bring them to environments with the proper oxygen content.
(9) Surface coatings: Bacterial cells protect themselves from environmental conditions with either an S layer or a glycocalyz, or both.
S layers are single alyers of thousands of copies of a single protein linked together like tiny chain link fences. Many different species have the ability to produce an S layer, including pathogens such as Clostridioides difficile and Bacillus anthracis. Some bacteria use S layers to aid in attachment as well.
Glycocalyx: The glycocalyx develops a coating of repeating polysaccharide or glycoprotein units. This protects the cll and in some cases helps it adhere to its evironment. Some bacteria are cvoered with a loose glycocalx shield called a slime layer that can protect them from loss of water and nutrients.
Glycolyces can be important in formation of biofilms. The thick, white plaque that forms on teeth comes in part from teh surface slimers produced by certain streptococci in the oral cavity. The slime protects them from being dislodged from the teeth and provides a niche for other oral bacteria that, in time, can lead to dental disease. The glycocalyx of some bacteria is so sticky that it has a significant role in the persistent colonization of nonliving materials such as palstic catheters, intraueterine devies and metal pacemakers that are in common medical use.
Capsules: A glycocalyx is called a capsule when it is bound more tightly to the cell than a slime layer is and it is denser and thicker. Capsules are often visible in negatively stained preparations and lead to a prominently sticky character to colonies on agar. Capsules are formed by many pathogenic bacteria, such as Streptococcus pneumoniae, Haemophilus influenzae (one cause of meningitis) and Bacilus anthracis (cause of antrax). Encapsulated bacterial cells generally have greater diase cuasing abilities because capsules protect bacteria against phagocytes.
The presence of a capsule is associated with virulence because it interferes with phagocytosis. Most capsules consist of repeating sequences of 2-3 sugars. This capsule encases an endospore which is an extremely resistent structure (contains calcium dipicolinate) that encases the genome. When conditions become favorable signals turn on genes that allows the spore to germinate and a return to the vegetative state.
Endospores are dormant bodies produced by bacteria such as Bacillus Clostridium and Sporosarcina. These bacteria can exist in two different forms; a vegetative cell and an endospore. The vegetative cell is metabolically active and growing. When environmental conditions become challenging, these bacteria will form endospores in a process called sporulation. The endospore exists initially inside the vegetative cell but eventually the cell disintegrates and the endospore is on its own. Both gram positive and gram-negative bacteria can form endospores but the medically relevant ones are all gram positive. The heat resistance of endospores is due to their high content of calcium and dipicolinic acid. The calcium dipicolinate in endosomes removes water which leaves the endospore very dehydrated making it less vulnerable to the efects of heat. Bacterial endospores are capable of surviving indefinitely and have been recovered from a 250 million year old salt crystal.
The resistance of endospores to antibiotics is of immense concern. The endospore’s most external layer, the exosporium, is composed primarily of proteins, polysacharides and lipids. Inward from the exosporium, the spore coat is composed primarily of proteins, which envelops the peptidoglycan cortex. Interior to the cortext, the highly lipid nature of the inner membrane renders the spore core impermeable to several chemical compounds. These protective layers work collectively to passively dehydrate the spore core and maintain a state of dormancy. Upon encountering favorable environmental conditions, suficial sensory mechanisms of variable sportes recognize specific chemical signals, known as germinants and initiate metabilic cascades that result in the germination and outgrowth of the spore to its vegetative form.
Spore-forming bacterial species reported as being pathogen to humans, livestock and insects below to the general Bacillus and Clostridium. Bacillus anthracis is the gaent of antrax and its ability to form endospores makes it an ideal condidate for bioterrorism. Teh genus Clostridium includes even more pathogens such as C. tetani, the cause of tetanus (lockjaw), C. perfringens, the cause of gas gangrene, and C. botulinu, the cause of botulism. Becasue they inhabit the soil and dust, endospores are constant intruders where sterility and leanliness are important. Hospitals take precautions to guard against Clostridioides difficile, the casue of a severe gastrointestinal disease known as C. diff and endospore destrcction is also a particular concern in the food canning industry. Ordinary boiling (100C) will usually not destroy such endospores, so canning is carried out in pressurized steam at 120C for 20-30 minutes.