Bacterial Shapes and Classification

Classification

There are thousands of species of bacteria on earth, many of which have not yet been identified.  When attempting to classify a bacterium, a variety of characteristics are used, including visual characteristics and laboratory tests.  Bacteria are simple, unicellular organisms. Most are free-living organisms, but a few require animal or plant hosts for survival. Bacteria absorb nutrients from their environments, excrete waste products, and secrete various toxins that help them invade tissues. Bacteria have no enclosed nucleus. Their chromosomal material is in the form of a large loop, packed into the cytoplasm of the cell.

Some bacteria can be identified through a simple visual perusal.  First, the operator considers the appearance of the bacterial colony (a group of the same kind of bacteria growing together, often on a petri dish.)  The operator also views individual bacteria under a microscope, considering their shape, groupings, and features such as the number and location of flagella. 

A variety of laboratory techniques can be used to narrow down the identity of a bacterial species if a visual survey is not sufficient.  The operator can stain the bacteria using a gram stain or an acid-fast stain.  The bacteria can be cultured on a specific medium which promotes the growth of certain species, as in the membrane filter method of testing for coliform bacteria.  Other tests can detect bacterial by-products, while yet more advanced tests actually analyze the DNA of the bacteria. 

Bacterial Shapes

The most basic method used for identifying bacteria is based on the bacterium's shape and cell arrangement.  This section will explain the three morphological categories which all bacteria fall into - cocci, bacilli, and spirilla.  You should keep in mind that these categories are merely a way of describing the bacteria and do not necessarily refer to a taxonomic relationship. The most common shapes of bacteria include rod, cocci (round), and spiral forms. Cellular arrangements occur singularly, in chains, and in clusters. Some species have one to numerous projections called flagella enabling the bacteria to swim, making them motile organisms.

Cocci (or coccus for a single cell) are round cells, sometimes slightly flattened when they are adjacent to one another.  Cocci bacteria can exist singly, in pairs (as diplococci ), in groups of

four (as tetrads ), in chains (as streptococci ), in clusters (as stapylococci ), or in cubes consisting of eight cells (as sarcinae .)

Bacilli (or bacillus for a single cell) are rod-shaped bacteria.  Since the length of a cell varies under the influence of age or environmental conditions, you should not use cell length as a method of classification for bacillus bacteria.  Like coccus bacteria, bacilli can occur singly, in pairs, or in chains.  Examples of bacillus bacteria include coliform bacteria , which are used as an indicator of wastewater pollution in water, as well as the bacteria responsible for typhoid fever. 

Spirilla (or spirillum for a single cell) are curved bacteria which can range from a gently curved shape to a corkscrew-like spiral.  Many spirilla are rigid and capable of movement.  A special group of spirilla known as spirochetes are long, slender, and flexible.

 

 

Gram Stain Procedure

The most fundamental technique for classifying bacteria is the gram stain, developed in 1884 by Danish scientist Cristian Gram. It is called a differential stain because it differentiates among bacteria and can be used to distinguish among them, based on differences in their cell wall.

In this procedure, bacteria are first stained with crystal violet, then treated with a mordant - a solution that fixes the stain inside the cell. The bacteria are then washed with a decolorizing agent, such as alcohol, and counterstained with safranin, a light red dye. The walls of gram-positive bacteria (ie. Staphylococcus aureus) have more peptidoglycans (the large molecular network of repeating disaccharides attached to chains of four or five amino acids) than do gram-negative bacteria. Thus, gram-positive bacteria retain the original violet dye and cannot be counterstained.

Gram-negative bacteria (ie. Escherichia coli) have thinner walls, containing an outer layer of lipopolysaccharide, which is disrupted by the alcohol wash. This permits the orignial dye to escape, allowing the cell to take up the second dye, or counterstain. Thus, gram-positive bacteria

stain violet, and gram-negative bacteria stain pink. The gram stain works best on young, growing populations of bacteria, and can be inconsistent in older populations maintained in the laboratory.

 

 

Respiration

Although we think of respiration as breathing, respiration is actually the process by which organisms break down organic substances (such as sugars) to produce energy.  All living organisms must perform some kind of respiration. 

In many cases, the chemical process of respiration requires oxygen, although some organisms are able to carry out respiration in the absence of oxygen.  This page will explain the three types of respiration found in microorganisms, as well as how these types of respiration affect the wastewater treatment plant. 

Aerobic Respiration

Aerobic respiration is respiration in the presence of oxygen.  Most multicellular organisms and many microorganisms produce their energy using aerobic respiration.  In aerobic respiration, sugars are broken down in the presence of oxygen to produce carbon dioxide, water, and energy.  Without oxygen, aerobic microorganisms are unable to produce energy and quickly die. 

Anaerobic Respiration

Other microorganisms are able to survive in environments which lack oxygen by performing anaerobic respiration , sometimes known as fermentation .  Like aerobic respiration, anaerobic respiration breaks down sugars and releases energy.  However, anaerobic respiration is typically slower and less efficient than aerobic respiration.   In addition, anaerobic respiration involves chemicals other than oxygen and carbon dioxide. 

The chemicals used and produced during anaerobic respiration vary from microorganism to microorganism.  Some anaerobic microorganisms use sulfate (SO4

2-) during respiration and produce hydrogen sulfide (H2S.)  Other microorganisms use nitrate (NO3

-), producing nitrite (NO2

-), nitrous oxide (NO), or nitrogen gas (N2 ).  Yet other microorganisms are able to use hydrogen gas (H2 ), producing methane (CH4 ) or acetic acid (CH3COOH-) as the byproduct. 

Anaerobic reactions generally lead to more offensive end products than those produced during aerobic respiration.  For example, hydrogen sulfide is very reactive and smells like rotten eggs

even at low concentrations.  Hydrogen sulfide can combine with the organic end products of anaerobic respiration to form the dark-colored, odorous substances which are characteristic of anaerobic (also known as septic ) conditions. 

Facultative Anaerobic Respiration

Many microorganisms are either obligate aerobes or obligate anaerobes.  That is, those which perform aerobic respiration will die if the oxygen content of their environment drops too low.  In contrast, those which perform anaerobic respiration will die if they are brought in contact with oxygen. 

The final type of microorganisms - facultative anaerobes - are able to perform either aerobic respiration or anaerobic respiration depending on the oxygen content of their environment.  Since aerobic respiration is more efficient, facultative anaerobes will perform aerobic respiration if there is oxygen present in their environment.  However, in the absence of oxygen, these organisms simply switch over to anaerobic respiration.  Coliform bacteria are a well-known example of facultative anaerobic microorganisms. 

In the Treatment Plant

In most wastewater treatment processes, operators attempt to maintain an environment suitable for aerobic respiration.  By maintaining an aerobic environment the operators prevent the bad smells associated with septic environments and also maintain a higher speed of waste digestion.  Aerobic processes are most common in biological wastewater treatment systems, including the activated sludge process, trickling filters, and many oxidation ponds. 

Since aerobic microorganisms use up oxygen as they break down waste, it is often necessary to aerate (add air to) the wastewater to maintain an aerobic environment.  Aeration may be achieved by blowing air into the water or (as in the trickling filter) by allowing water to run through the air. 

Despite the advantages of aerobic systems, some wastewater treatment processes are designed to be anaerobic.  Both anaerobic digesters and septic tanks are wholly anaerobic environments.  Since these systems house obligate anaerobic microorganisms, exposing anaerobic digestion systems to oxygen even for a short period of time can seriously affect the systems' ability to function. 

Some systems, through accident or design, can function both aerobically and anaerobically.  Although oxidation ponds are generally aerobic, bottom deposits and stagnant pockets in the ponds often become anaerobic.  On a trickling filter's slime layer, aerobic and anaerobic zones may occur within millimeters of each other, with the surface layers being aerobic and the deeper layers being anaerobic.

Facultative anaerobic microorganism species are very important in many wastewater treatment processes since they area able to perform in both aerobic and anaerobic environments.  However, facultative anaerobic microorganisms can cause problems when  they begin to respire anaerobically, producing unpleasant byproducts.  In general, facultative anaerobic species usually begin performing anaerobic reactions when the dissolved oxygen levels of their environment fall below about 0.5 mg/L for several hours.  This condition is rarely met in aeration tanks unless equipment failure occurs, but keeping sludge in the final clarifiers for an extended period of time can lead to anaerobic conditions.  In addition, conditions of low flow or elevated temperature can result in anaerobic conditions. 

 

 

Bacterial Cell StructureInternal Structure: Bacteria have a very simple internal structure, and no membrane-bound organelles.

nucleoid

DNA in the bacterial cell is generally confined to this central region. Though it isn't bounded by a membrane, it is visibly distinct (by transmission microscopy) from the rest of the cell interior.

ribosomes

Ribosomes give the cytoplasm of bacteria a granular appearance in electron micrographs. Though smaller than the ribosomes in eukaryotic cells, these inclusions have a similar function in translating the genetic message in messenger RNA into the production of peptide sequences (proteins).

storage granules

(not shown) Nutrients and reserves may be stored in the cytoplasm in the form of glycogen, lipids, polyphosphate, or in some cases, sulfur or nitrogen.

endospore

(not shown) Some bacteria, like Clostridium botulinum, form spores that are highly resistant to drought, high temperature and other environmental hazards. Once the hazard is removed, the spore germinates to create a new population.

Surface Structure: Beginning from the outermost structure and moving inward, bacteria have some or all of the following structures:

capsule

This layer of polysaccharide (sometimes proteins) protects the bacterial cell and is often associated with pathogenic bacteria because it serves as a barrier against phagocytosis by white blood cells.

outer membrane

(not shown) This lipid bilayer is found in Gram negative bacteria and is the source of lipopolysaccharide (LPS) in these bacteria. LPS is toxic and turns on the immune system of , but not in Gram positive bacteria.

cell wall

Composed of peptidoglycan (polysaccharides + protein), the cell wall maintains the overall shape of a bacterial cell. The three primary shapes in bacteria are coccus (spherical), bacillus (rod-shaped) and spirillum (spiral). Mycoplasma are bacteria that have no cell wall and therefore have no definite shape.

periplasmic (not shown) This cellular compartment is found only in those

space

bacteria that have both an outer membrane and plasma membrane (e.g. Gram negative bacteria). In the space are enzymes and other proteins that help digest and move nutrients into the cell.

plasma membrane

This is a lipid bilayer much like the cytoplasmic (plasma) membrane of other cells. There are numerous proteins moving within or upon this layer that are primarily responsible for transport of ions, nutrients and waste across the membrane.

Appendages Bacteria may have the following appendages:

pili

These hollow, hairlike structures made of protein allow bacteria to attach to other cells. A specialized pilus, the sex pilus, allows the transfer of plasmid DNA from one bacterial cell to another. Pili (sing., pilus) are also called fimbriae (sing., fimbria).

flagella

The purpose of flagella (sing., flagellum) is motility. Flagella are long appendages which rotate by means of a "motor" located just under the cytoplasmic membrane. Bacteria may have one, a few, or many flagella in different positions on the cell.

Shapes of bacteriaThe shape of bacterial cells is of fundamental importance in the classification and identification of bacteria. The majority of bacterial cells come in three basic shapes: round, rod shaped, or spiral. However, they display a remarkable variety of forms when viewed microscopically: Types of bacteria: (1) cocci, (2) diplococci, (3) streptococci, (5)

bacilli. Some bacteria possess hairlike flagella, for example (6) flagellate rods or (7) flagellate spirilla. At (4) a bacillus is shown undergoing reproduction by binary fission.

1. Round (spherical) bacteria are referred to as cocci (singular: coccus).2. Elongated or rod-shaped cells are known as bacilli (singular: bacillus).3. Ovoid cells are something in between cocci and bacilli. These are known as coccobacilli (singular

coccobacillus). 4. Spiral shaped cells can be one of two types: either rigid called spirilla (singular spirillum) or

flexible called spirochaetes (singular spirochaete). Spiral-shaped bacteria are distinguished by their length, the number and size of the spirals, and direction of the coil. Short segments or incomplete spirals are common, as the comma-shaped Vibrios. The spirochetes of syphilis are typical spiral bacteria. Diseases caused by spirochaetes include the following: syphilis, yaws, leptosporosis, and Lyme disease.

5. Square bacteria are flat and box-like, but can vary in their angular shape. 6. There are also fungal bacteria, known as actinomycetes, which grow like fine threads, called

hyphae (singular hypha). A mass or group of these is known as 'mycelium'. One example is Actinomyces scabies, which resembles fungal mycelia. Specialized reproductive elements produce conidia (functioning similar to spores) that are eventually released into the air.

Structure of bacteria

Cell envelop

• The cell envelope is all the layers from the cell membrane outward, including the cell wall, the periplasmic space, the outer membrane, and the capsule.

– All free-living bacteria have a cell wall

– periplasmic space and outer membrane are found in Gram-negatives

– the capsule is only found in some strains

All cells have 3 main components:DNA (‘nucleoid”)genetic instructionssurrounding membrane (“cytoplasmic membrane”)limits access to the cell’s interiorcytoplasm, between the DNA and the membranewhere all metabolic reactions occurespecially protein synthesis, which occurs on the ribosomesBacteria also often have these features:cell wallresists osmotic pressureflagellamovementpiliattachmentcapsuleprotection and biofilms

Cell Membrane

Cell wall

The cell membrane (often called the plasma membrane) is composed of 2 layers of phospholipids.Phospholipids have polar heads and non-polar tails. “Polar” implies that the heads are hydrophilic: they like to stay in an aqueous environment: facing the outside world and the inside of the cell.“non-polar” means that the tails are hydrophobic: they want to be away from water, in an oily environment. The tails are in the center of the membrane

A pure phospholipid membrane only allows water, gasses, and a few small molecules to move freely through it.

• Osmotic pressure is the force generated by water attempting to move into the cell.

– Water can go through the cell membrane freely

– The contents of the cell are very concentrated

– Like all things, water moves from areas of high concentration to areas of low concentration. This means, water will move from outside the cell (dilute environment) to inside (concentrated environment).

– Osmotic pressure can easily cause a cell to swell up and burst.

• Bacteria, along with plants and fungi, resist osmotic pressure by surrounding the cell in a rigid box, the cell wall.

– Composed of peptidoglycan (also called proteoglycan or murein)

– Long chains of polysaccharide cross-linked by short peptides (amino acid chains).

• The peptides contain the unusual mirror-image amino acids D-alanine and D-glutamate

• polysaccharide is composed of alternating “amino sugars”: N-acetylglucosamine and N-acetylmuramic acid

Capsule : some bacteria (often pathogens) are surrounded by a thick polysaccharide capsule. This is a loose jelly-like or mucus-like layer. It helps prevent immune system cells from reaching the bacteria, and it forms part of biofilms.

Pili (singular = pilus) are hairs projecting from the surface. They are composed of pilin protein. There are several types:DNA can be transferred between bacteria by conjugation, which is initiated when sex pili on the donor cell attach to and draw in the recipient cell.Fimbriae (singular = fimbria) are pili used to attach the bacteria to target cells ( in infection) or to surfaces, where they form a biofilm.

Flagella are long hairs used to propel the cells. They are composed of flagellin protein.At the base of each flagellum is a motor embedded in the membrane and cell wall. It turns in a rotary motion, driven by proton-motive force (the flow of protons i.e. H+ ions across the cell membrane).The suffix “-trichous” is used to describe the placement of flagella: e.g. lophotrichous = several flagella all clustered at one end.