Chapter 1 - Structure of Prokaryotic Cells PDF

**Chapter 1: Structure of prokaryotic cells** +-----------------------------------------------------------------------+ | **[Learning Objectives]** | | | | By the end of this section, you should be...

**Chapter 1: Structure of prokaryotic cells** +-----------------------------------------------------------------------+ | **[Learning Objectives]** | | | | By the end of this section, you should be able to: | | | | a. Characterise the structure of prokaryotic cells. | | | | b. Differentiate between Gram positive and Gram negative bacterial | | cells. | +-----------------------------------------------------------------------+ **Introduction** Microbiology is the study of tiny organisms that cannot be seen with the unaided eye. In other words, they are microscopic and can only be viewed when magnified by a microscope. These organisms are collectively known as microorganisms or microbes. The unaided human eye cannot visualise things that are smaller than 1 mm. To provide a quick size comparison, Figure 1 shows the relative sizes of some objects. The microbes normally encountered in the study of microbiology tend to fall within the micrometer (µm) dimensions (Figure 1), with viruses being the smallest microbes. Three groups of micro-organisms will be of focus in this module: bacteria, fungi and viruses. Viruses are not living organisms but they are considered a type of microbe as they are microscopic and can cause infections and disease. They are small particles that exist at a level of complexity between large molecules and cells. Viruses are in fact simpler than cells; they are in fact composed essentially of a protein covering that wraps around a small amount of nucleic acid (DNA or RNA). Despite their simple make-up, viruses have the ability to invade host cells and can inflict serious damage and death. Thus, microbes are found everywhere. Some are harmful, but most are beneficial. Their presence and biological activities contribute tremendously to the well-being of living things and their environment. **\ ** **Classification of Microbes (INFO)** Living organisms can be systematically organised using many different classification systems. Classification is an orderly arrangement of organisms into groups that indicate their evolutionary relationships and history. The main taxa (or groups) in a classification scheme begin with **domain** (which is a giant, all-inclusive category based on a unique cell type), and ends with **species**, the smallest and most specific taxon (pl. taxa). All the members of a domain share only one or few general characteristics, whereas members of a species - essentially the same kind of organism, share many characteristics. The order of taxa between the top and bottom levels is (in descending order): domain, kingdom, phylum (or division), class, order, family, genus and species. Using molecular methods, Carl Woese and George Fox have classified living organisms into three domains. The members of Domain Bacteria have prokaryotic cells and are what people think of as traditional bacterial species. Members of the Domain Archaea also possesses prokaryotic cells, but these cells are so distinct from those found in the Bacteria that they are placed in their own domain. Most members of this domain are characterised by their ability to live in extreme environments (E.g. deserts, volcanoes, the Antarctic etc.) or produce novel metabolic by-products (E.g. methane). Biologists have estimated that these two domains together account for at least half of the total mass of life forms on earth. This is largely due to their versatility and adaptability to a variety of habitats. The Domain Eukarya contains all of the organisms that display an eukaryotic cell structure, and includes Kingdom Fungi, Kingdom Protista, Kingdom Animalia and Kingdom Plantae. These are more commonly known as fungi, protists, animals and plants, respectively. **Binomial Naming of Organisms** Scientists use a two-name (binomial) system to name all organisms on Earth. This system describes the *genus* and *species* of the organism. For example, Humans are scientifically named *Homo sapiens*. The first word is the **genus** name and the second is the **species** name. [There are four rules that must be applied when naming an organism]: 1. The genus name is always written first, followed by the species name. 2. The genus name is always capitalized, the species name is not. 3. Both names must be underlined separately; alternatively, they are not underlined but written in italics. 4. The genus name may be represented by its first letter, but this is not applicable to the species name.  Here is an example: **Binomial name of organism** **Abbreviation** **Remarks** ------------------------------- ------------------ -------------------------------------------------------------------------------------------------------------------------------------------------------------------------- *Escherichia coli* *E. coli* Some strains can cause severe food poisoning *Staphylococcus aureus* *S. aureus* Can cause skin infections when it enters open wounds *Bacillus cereus* *B. cereus* Can cause 'Fried Rice Syndrome'. This is classically contracted from consumption of fried rice that have been sitting at room temperature for hours (such as at buffets) **Cell structure** A cell is the basic unit of all living organisms that is capable of independent existence. Cells are highly complex and organised. They contain DNA to construct cellular structures, run cellular activities and reproduce. Cells also carry out basic cellular functions that are essential for survival and growth. There are two basic cell types, **prokaryotic** and **eukaryotic**. They can be distinguished by their size and types of cellular structures possessed. Note that all prokaryotes are microbes while only some eukaryotes are microbes. A prokaryotic cell is usually smaller than a eukaryotic cell. It does not have any membrane-bound genetic material and structures while a eukaryotic cell has its genetic material enclosed by the nuclear membrane (forming the nucleus) and many membranous structures. The genetic material possessed by prokaryotic cells is usually a double stranded circular chromosome while that of eukaryotic cells are typically found in more than 1 paired chromosomes. A eukaryotic cell also has very complex internal structure and possesses many structures lacking in prokaryotic cells. Some examples are listed below in Table 1. Table 1: Comparison of some prokaryotic and eukaryotic cellular structures **Structures** **Eukaryotic cell (eg. fungi)** **Prokaryotic cell (eg. bacteria)** ------------------------------------------------------------------ ------------------------------------------------------------ ------------------------------------------------------------------------- Nucleus/nuclear region Nucleus (linear chromosomes are bound by nuclear membrane) Nucleoid (single, circular chromosome is not bound by nuclear membrane) Ribosomes Many, larger Less, smaller Cell membrane, cytoplasm Yes Yes Membrane-bound organelles (eg. ER, mitochondria, lysosomes etc.) Yes No Two generalised cells (bacterial and fungal cell) can be used to understand more about cellular structures, and their corresponding functions. Refer to Figures 2-3.  Being prokaryotic cells, bacterial cells are have a much less complex structure than fungal cells (which are eukaryotic). Table 2 shows a brief comparison of cell structures. Table 2: Comparison of cellular structures within bacterial and fungal cells \*note: while fungi are generally non-motile, but some classes of fungi do produce motile spores. **Bacterial structure** Bacteria are unicellular prokaryotes. They reproduce by binary fission (will be covered in greater detail in Chapter 2). Bacteria are relatively small, with sizes ranging from 0.5-1.0 µm in diameter. Their small size gives them a large surface area to volume ratio, enabling large surfaces of the bacteria cell to be in contact with the environment. A bacterial cell is made up of several generalised structures (Figure 4).  Figure 4: Bacterial structures (Ref: K. P. Talaro. Foundations in Microbiology. 5^th^ Edition. 2005. McGraw-Hill, Inc. USA. pp 88.) This flowchart summarises the general structural plan of a typical bacterial cell: **\ ** **External structures** Very often, bacteria may have surface structures (called **appendages**) arising from their surfaces. These can be divided into two groups: those that provide motility (E.g. flagella and axial filaments); and those that provide attachment sites (E.g. fimbriae) or channels (E.g. pili). **Fimbria** (pl. fimbriae) and **Pilus** (pl. pili); refer to Figure 5. - - -  **Flagella** (sing. flagellum) They are long, thin, rounded structure that rotates 360^o^ (Figure 6). This is in contrast to the flagella of eukaryotic cells which undulate back and forth. Flagella confer cell motility (allows bacterial cells to swim freely in aquatic habitats) and are made from flagellin protein. All spirilla, about half of the bacilli and a small number of cocci are flagellated (bear flagella). Flagella vary in number and arrangement on a cell as shown below. Figure 6: Different types of flagellar arrangements (or flagellation) Motile bacteria may be propelled by one or more flagella per cell as shown above. On the other hand, eukaryotic cells may be able to move either with the help of flagella (eg. human sperm cells) or **cilia** (eg. protists such as *Paramecium* or *Euglena*). The ability to move is one piece of information used in the laboratory identification of various groups of bacteria. Special stains must be used to see flagella arrangement because they are usually to minute to be seen with a light microscope. One way to detect for bacterial motility is to stab a tiny mass of cells into a test-tube of semi-solid agar medium. Rapidly spreading growth is indicative of cell motility. You will be performing this experiment in the lab. Motile cells have the ability to respond to environmental signals such as chemicals. This is known as **chemotaxis**. Positive chemotaxis is the movement of a cell towards a favourable chemical stimulus (E.g. a food source); whereas negative chemotaxis is the movement away from a repellent (E.g. potentially harmful compound). A few pathogenic (disease-causing) bacteria use their flagella to invade the surface of mucous membranes during infections. *Helicobacter pylori*, the agent of stomach ulcers, bores through the stomach lining, and *Vibrio cholerae*, the cause of cholera, penetrates the small intestine with the help of flagella. **Glycocalyx** The bacterial cell surface is often subjected to severe environmental conditions. Some (not all!) bacterial cells can secrete some extracellular material as a form of protection. The **glycocalyx** (Figure 7) develops as a coating of macromolecules to protect the cell and in some cases, help it to adhere to its environment. Glycocalyces differ in thickness, organisation and composition. Some bacteria are covered with a loose shield called a **slime layer** that protects them from dehydration and loss of nutrients, as well as serving in adhesion. Other bacteria produce **capsules** that are attached tightly to the bacterial cells and have a more defined shape. Capsules are composed of repeating polysaccharide units of protein, carbohydrate or both. Capsules have a thick, gummy consistency that gives a sticky (mucoid) appearance to the colonies of encapsulated bacteria. Capsules prevents host cells from mounting an immune response because bacteria with capsules can resist engulfment and destruction by white blood cells. Cell envelope and glycocalyces Figure 7: Two types of glycocalyces (sing. = glycocalyx) (Source: Chess, B. Talaro's Foundations in Microbiology. 11^th^ Edition. 2021. McGraw-Hill, Inc. USA. pp 98.) **Cell envelope (consists of cell membrane & cell wall)** Most bacteria have an extensive cell envelope (making up a tenth to up to half of the cell volume) comprising of three layers stacked on top of each other. Each of these layers performs a distinct function, but together they act as single protective unit. **Cell wall** - Surrounds the cell membrane - Strong and semi-rigid casing that provides structural support and maintains cell shape - Protects cell against changes in osmotic pressure - Many bacteria have porous cell walls so the cell walls are not as important as cell membranes in regulating the entry of material into and out of cells - Primarily made up of **peptidoglycan** (Figure 8)  Figure 8: Structure of peptidoglycan, also known as murein, is a polymer consisting of sugars (glycan) and amino acids (peptido) that forms a mesh-like layer external to the cell membrane. The sugar portion consists of alternating residues of β, 14 linked NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid). NAM molecules from neighbouring glycan chains are held together by branches of four amino acids (called peptide cross links). The bacterial cell wall surrounds the entire bacterium, holding the cell together and offering protection. It also maintains osmotic pressure, meaning it lets in just the right amount of water, sugars, amino acids and other ions that the cell needs. This prevents the cell from lysis (bursting). Bacteria are grouped based on differences in their cell wall structures as seen after Gram staining has been performed (Figure 9).  Figure 9: Gram staining procedure. Upon completion, Gram positive cells would appear dark purple (or blue), while Gram negative cells would appear red (or pink). Ref: Talaro & Chess. Foundations in Microbiology. 10^th^ Edition. 2018. McGraw-Hill, Inc. USA. The peptidoglycan layer of Gram positive bacteria can be up to 30 sheets of glycan chains thick. On the other hand, that of Gram negative bacteria is only one or two sheets thick (Figure 10). Gram positive bacterial cell walls have a thick peptidoglycan layer (Table 3) which retains Gram Stain crystal violet. There is a thin (almost negligible) periplasmic space in between cell wall and cell membrane. There may also be additional molecules, teichoic acid and lipoteichoic acid to add rigidity to the cell wall and are also essential for survival/ virulence of bacterial pathogens (allows them to attach to host tissues to cause illness when released into bloodstream). Gram negative bacterial cell walls have a much thinner peptidoglycan layer that does not retain Gram Stain crystal violet well. The peptidoglycan layer is surrounded by an additional layer (called the 'outer membrane') consisting of lipopolysaccharide and phospholipid. This additional layer protects the cells from antibiotics and enzymes. They also have a thick periplasmic space between the cell wall and cell membrane. cow95289\_04\_14 Figure 10: Cell wall structural comparison of Gram positive and Gram negative bacteria (Ref: Talaro & Chess. Foundations in Microbiology. 10^th^ Edition. 2018. McGraw-Hill, Inc. USA. pp 101.) Table 3: Key differences between the cell walls of Gram positive and Gram negative bacteria (Fill in the blanks using clues given during your lecture) +-----------------------+-----------------------+-----------------------+ | Characteristics | Gram positive | Gram negative | | | bacteria | bacteria | +=======================+=======================+=======================+ | Thickness of | Thick; 60% of CW; | Thin; 10-20% of CW; | | peptidoglycan layer | 20-80 nm | 8-11 nm | +-----------------------+-----------------------+-----------------------+ | Outer membrane | Absent | Present (together | | | | with the periplasmic | | | | space, protects | | | | against antibiotics & | | | | enzymes) | +-----------------------+-----------------------+-----------------------+ | Chemical composition | Peptidoglycan | Lipopolysaccharides | | | | (LPS) | | | Teichoic acid | | | | | Lipoprotein | | | Lipoteichoic acid | | | | | Peptidoglycan | | | Mycolic acid & | | | | polysaccharides\* | Porin proteins | +-----------------------+-----------------------+-----------------------+ | Periplasmic space | Thin | Thick | +-----------------------+-----------------------+-----------------------+ | Permeability to | More penetrable | Less penetrable | | molecules | | | +-----------------------+-----------------------+-----------------------+ \*only in acid-fast bacteria (E.g. *Mycobacterium* species) Porins are channel proteins that allow the passage of useful, small, hydrophilic molecules (nutrients) across the outer membrane, but they prevent the passage of larger, harmful substances like bile salts (commonly present in the human intestine). This protects the bacteria and allows them to live in our dastrointestinal tract.. **Cell membrane** While the glycocalyx (if present) and cell wall can bar the passage of large molecules, it is the cell membrane that acts as a barrier between the inside and outside of the cell. Here are its key features: - Selectively-permeable, flexible thin sheet that surrounds the cytoplasm - Comprises of a bilayer of phospholipids (30-40%) and proteins (60-70%). The membranes of Archaea which contain unique, branched hydrocarbons, rather than fatty acids - Most important function of cell membrane is to regulate transport -- the passage of nutrients into the cell and discharge of wastes - As bacterial cells do not have any membrane-bound organelles, the cell membrane provides a site for energy reactions, nutrient processing and synthesis **Cytoplasm** The site for many of the cell's biochemical and synthetic activities. It is mostly made up of water (70-80%), which serves as a solvent for nutrients (E.g. sugars, amino acids, salts etc.). The cytoplasm holds larger structures such as the chromosome, ribosomes, granules, plasmids etc. **Nucleoid** By definition, bacteria do not have a true nucleus. Their DNA is not enclosed by a nuclear membrane but is instead condensed in a central area of the cell called the **nucleoid** (or **nuclear region**). Bacterial DNA occurs in the form of a single, circular strand. Arranged along its length are genes that carry information required for bacterial growth and reproduction. Although the bacterial chromosome contains all the essential information for bacterial survival, many bacteria contain additional genetic elements called **plasmid**. These are small pieces of circular DNA that exist independently within the cytoplasm. At times, they may be integrated with the nucleoid. During bacterial reproduction, plasmids are duplicated and passed on to the offspring. Plasmids are not essential for growth or metabolism, but can carry genes that confer protective traits like antibiotic resistance or toxin production. Some yeasts (eukaryotes) may also have plasmids. Plasmids are an important component of genetic engineering techniques because they are readily manipulated in the lab for the purpose of transferring genes into bacterial cells. **Ribosomes** Each bacterial cell contains tens of thousands of **ribosomes**. They are the sites of protein synthesis. Ribosomes are made from protein [and] ribosomal RNA. Each ribosome consists of a large subunit and a small subunit. In the cytoplasm, ribosomes can occus in chains (polysomes) or they can be attached to the cell membrane. Ribosomes can be characterised in the lab by Svedberg (S) units. This rates the molecular sizes of various cellular structures when they are spun down using a lab centrifuge. Heavier, more compact structures sediment faster and are given a higher S rating. Prokaryotic ribosomes are fewer and smaller (70S), whereas eukaryotic ribosomes are more plentiful and larger (80S). Being prokaryotic cells, bacteria do not possess any membrane-bound organelles like mitochondria, smooth- and rough- ER, Golgi apparatus etc. **Endospores** Some bacteria have a two-phase life cycle: a vegetative cell (metabolically active, growing organism) and an **endospore** (inert, dormant body). During adverse environmental conditions, (E.g. nutrient depletion, dehydration etc.), these bacteria are induced to produce endospores through a process known as **sporulation** (Figure 11). Each cell can only produce one endospore. Endospores are very difficult to kill as they able to withstand extremes of heat, drying, freezing, radiation and the chemicals that would readily kill ordinary cells. Figure 11: Formation of bacterial endospore ('spore') during environmental stress; and spore germination when favourable conditions return. Once conditions become favourable, each endospore will undergo **germination** to form a new vegetative cell, thus enabling the bacteria to survive. Once initiated, germination can occur as soon as 90 minutes later. The stimulus is usually water, and a specific germination agent (E.g. an amino acid or an inorganic salt). To date, two rod-shaped, Gram positive bacterial genera are known to have the ability to produce endospores: *Clostridium* and *Bacillus*. In the laboratory, endospore staining is carried out using malachite green to stain the endospore, and safranin as a counterstain. **[Bacterial morphology (shape) ]** Bacterial cells function as independent, single-celled (unicellular) organisms. They exhibit considerable variety in shape, size and colonial arrangement. It is most convenient to describe most bacteria by one of three general morphologies as designated by the shape of the cell wall (Table 4). If the cell is spherical, the bacterium is described as a **coccus** (pl. cocci). Cocci are often perfect spheres, but they can also appear slightly oval or bean-shaped. A cell that is cylindrical is termed a rod, or **bacillus** (pl. bacilli). There is also a bacterial genus named *Bacillus* whereby cells are all rod-shaped. When a rod-shaped cell is slightly elongated, it is termed a **coccobacillus** or short-rod (length not exceeding 2 µm). If it is gently curved or comma-shaped, it is termed a **vibrio**. Bacteria that have a spiral-shaped cylinder are called **spirillum** (pl. spirilla). They consist of a rigid helix, twisted 1-20 times along its axis (like a corkscrew) and may have one to several flagella attached at one end of the cell. Another spirally-shaped cell that was mentioned earlier is the **spirochete**. This is a more flexible form that resembles a spring as it has 3-70 helical turns. Spirochetes move with the help of between 2-100 periplasmic flagella (also called axial filaments). Table 4: Common bacterial morphologies +-----------------------+-----------------------+-----------------------+ | Morphology | Description | Example (no need to | | | | memorise) | +=======================+=======================+=======================+ | Bacillus | Rod-shaped cells | *Bacillus* species | | | whose dimensions vary | are about 2 µm in | |  | is a genus name while | *Escherichia coli* | | | the term bacillus | cells are typically 1 | | | also describes a | µm in width and 2-3 | | | shape. | µm in length. | +-----------------------+-----------------------+-----------------------+ | Coccus | Describes any | | | | bacterium that has a | | | File:Arrangement of | spherical shape. | | | cocci bacteria.svg | Sometimes, they can | | | | occur as groups of | | |  | given names. | | | | | | | File:Arrangement of | | | | cocci bacteria.svg | | | | | | | |  | | | | | | | | File:Arrangement of | | | | cocci bacteria.svg | | | | | | | |  | | | +-----------------------+-----------------------+-----------------------+ | | Diplococci are | *Neisseria | | | arranged in 2-cell | gonorrhoeae* is an | | | pairs of small cocci, | example and it is the | | | usually joined along | causative agent for | | | their longest axis | gonorrhoea. | | | and with adjacent | | | | sides flattened. | | +-----------------------+-----------------------+-----------------------+ | | Bacteria that are | | | | typically arranged in | | | | clusters of 4 cells. | | +-----------------------+-----------------------+-----------------------+ | | Bacteria in the | | | | *Sarcina* genus | | | | typically form a | | | | cuboidal arrangement | | | | of 8-64 cells. | | +-----------------------+-----------------------+-----------------------+ | | Bacteria in the | *Streptococcus | | | *Streptococcus* genus | pyogenes*, which can | | | are arranged in | cause sore throats | | | chains. They divide | and also skin and | | | in one plane only and | soft tissue | | | the degree of | infections is an | | | adhesion between | example. The cells | | | cells is not very | are about 1-2 µm in | | | strong, thus the | diameter. | | | chains are easily | | | | broken. | | +-----------------------+-----------------------+-----------------------+ | | Those found in | *Staphylococcus | | | irregular clusters of | aureus* is often | | | cells (like grapes) | found as part of the | | | are called | normal flora of skin | | | staphylococci. Cell | and nostrils, but can | | | division takes place | also give rise to | | | in a number of planes | skin infections when | | | with a high degree of | there are open | | | cell adhesion. | wounds. Cells are | | | | typically 1 µm or | | | | less in diameter. | +-----------------------+-----------------------+-----------------------+ | Coccobacillus | A shape that is | *Escherichia coli* is | | | intermediate between | a Gram negative | | Bacteria shapes | cocci (spherical) and | coccobacillus | | | bacilli (rod-shaped). | commonly found in | | | Thus, they are very | human intestines. | | | short rods and are | Some strains are | | | often mistaken for | responsible for food | | | cocci. | poisoning, urinary | | | | tract infection etc. | +-----------------------+-----------------------+-----------------------+ | Vibrio | Rigid, curved cells | *Vibrio cholerae* | | | | (causative agent of | |  | | 0.5 µm and a length | | | | of 2 µm. | +-----------------------+-----------------------+-----------------------+ | Spirillum | Cells are made up of | *Helicobacter pylori* | | | rigid spirals with | (implicated in | | Bacteria shapes | 1-20 helical turns. | gastric ulcer | | | They resemble | formation). | | | corkscrews | Dimensions are | | | | typically 0.5 µm in | | | | diameter with | | | | variable length up to | | | | 60 µm. | +-----------------------+-----------------------+-----------------------+ | Spirochete | Thin, flexibly-coiled | *Treponema pallidum* | | | cells with between | (causative agent of | |  | | diameter is 0.5 µm by | | | | 5-500 µm length. | +-----------------------+-----------------------+-----------------------+ **[Bacterial arrangement ]** Bacteria also exhibit different cell arrangements (distinctive grouping of cells), depending on the division pattern and how the cells remain attached after division (Table 5). At times, individual cells may remain attached to others after cell division, thereby appearing in groups or clusters of cells. This is known as bacterial arrangement (or style of grouping). Thus, the main factors influencing bacterial arrangement are its pattern of cell division and how the cells remain attached afterward. The greatest variety in arrangement occurs among the spherically-shaped (**cocci**) bacteria (Figure 12). They may exist as singles, in pairs (**diplococci**), in **tetrads** (groups of four), in irregular clusters (**staphylococci**) or in chains of a few to hundreds of cells (**streptococci**). An even more complex grouping of a cubical packet of 8, 16 or more cells is called a **sarcina** (pl. sarcinae). These different coccal groupings is the result of cell division of a coccus in a single plane, in two perpendicular planes, or in several intersecting planes (after division, daughter cells remaining attached). Rod-shaped (**bacilli**) cells are less varied because they only divide in the transverse plane (perpendicular to long axis). Thus, they can be arranged as single cells, as a pair of cells with ends attached (**diplobacilli**), or as a chain of several cells (**streptobacilli**) (Figure 13).  Table 5: Bacterial morphologies and arrangements **Morphology** **Division without separation produces:** **Prefix** **Cell numbers** **Arrangement** **Morphology + Arrangement** ---------------- ------------------------------------------- ------------ ------------------ ----------------- ------------------------------ Coccus Single cells NA 1 Single Coccus Cells in pairs Diplo- 2 Pair Diplococci Cells in chains Strepto- Variable Chain Streptococci Four cells in a cube Tetrad 4 Tetrad Tetrads Cells in a cube Sarcina 8-64 Sarcina Sarcina (plural: sarcinae) Irregular cluster Staphylo- Variable Cluster Staphylococi Bacillus Single cells NA 1 Single Bacillus Cells in pairs Diplo- 2 Pair Diplobacilli Cells in chains Strepto- Variable Chain Streptobacilli Spirillum Single cells NA 1 Single Spirillum Here are some ways in which bacteria may be described: +-----------------------+-----------------------+-----------------------+ | **Bacterium** | **Description 1** | **or Description 2** | +=======================+=======================+=======================+ | | Morphology: coccus | staphylococcus | | | | | | | Arrangement: cluster | | +-----------------------+-----------------------+-----------------------+ |  | (or rod) | | | | | | | | Arrangement: chain | | +-----------------------+-----------------------+-----------------------+ | | Morphology: | Coccobacillus | | | coccobacillus (or | | | | short rod\*) | | | | | | | | Arrangement: single | | +-----------------------+-----------------------+-----------------------+ \*Less than 2 μm in length