Bacterial Genomes, Plasmids, Growth, and Genetic Mapping

This guide covers the structural, biochemical, physiological, and genetic principles of bacteria. It details the molecular organization of the bacterial genome, plasmid biology, nutritional classifications, growth kinetics, horizontal gene transfer (HGT), and the mathematical/experimental methods used to map bacterial chromosomes.

1. The Bacterial Genome and Chromosome Architecture

Structural Organization

The bacterial chromosome is primarily a circular, double-stranded DNA (dsDNA) molecule. Unlike eukaryotes, bacteria lack basic histone proteins. Instead, their DNA is complexed with low-molecular-weight polyamines and specialized nucleoid-associated proteins (NAPs) that facilitate compaction and structural regulation.

The chromosome is organized into independent, supercoiled loops called domains, allowing different parts of the genome to undergo structural changes during replication, transcription, and segregation simultaneously. The region containing this condensed DNA is called the nucleoid, which lacks a limiting nuclear membrane.

                    Bacterial Chromosome Domain Compaction

                               ,-'""`-.
                             ,'  _   _ `.
                            /   ( ) ( )  \   <--- Independently
                           |   _`-' `-'_  |       supercoiled domains
                           |  ( )     ( ) |       (loops)
                           |   `-' _ `-'  |
                            \     ( )    /   <--- Central protein scaffold
                             `.    `'  ,'        (NAPs and polyamines)
                               `-.____.-'
  

Nucleoid-Associated Proteins (NAPs)

NAPs act as functional analogues of eukaryotic histones by bending, bridging, wrapping, and condensing bacterial DNA:

• HU (Heat-Unstable Nucleoid Protein): One of the most abundant NAPs. It primarily functions by bending DNA, facilitating site-specific recombination, DNA repair, and initiation of replication.
• IHF (Integration Host Factor): Binds to specific DNA sequences and introduces a sharp bend of over 120 degrees. Unlike HU (which bends straight DNA), IHF often targets sequences that are already pre-bent, playing a critical role in integration, inversion, and transposition events.
• H-NS (Histone-like Nucleoid Structuring Protein): Functions in condensing DNA by bridging parallel DNA segments. It serves as a major transcriptional repressor, particularly targeting and silencing foreign genes acquired via horizontal gene transfer (xenogeneic silencing).
• SMC (Structural Maintenance of Chromosomes): Uses ATP hydrolysis to bridge distant DNA loci, facilitating the compaction of chromosomal arms and driving active chromosome segregation during division.

Genomic Architecture and Diversity

While the model bacterium Escherichia coli contains a single circular chromosome of approximately 4.6 Mbp, bacterial genomes exhibit significant structural diversity:

• Vibrio cholerae: The causative agent of cholera contains two circular chromosomes (Chromosome I is approximately 2.9 Mbp and Chromosome II is approximately 1.1 Mbp). Chromosome I carries most of the essential housekeeping and virulence genes, while Chromosome II contains metabolic genes and is thought to have evolved from a captured megaplasmid.
• Borrelia burgdorferi: The causative agent of Lyme disease possesses a single linear chromosome of approximately 1 Mbp accompanied by up to 21 linear and circular plasmids. Because its chromosome is linear, Borrelia cannot supercoil it into a tight nucleoid ball; instead, its genomic strands remain diffused throughout the cytoplasm.
• Agrobacterium tumefaciens: Contains one linear chromosome (approximately 2.1 Mbp) and one circular chromosome (approximately 3.0 Mbp), alongside massive plasmids (such as the Ti plasmid).

Archaeal Genomes: The Evolutionary Bridge

The DNA molecules of Archaea share characteristics of both bacteria and eukaryotes. Archaean genomes are circular and packaged into nucleoids, but they use eukaryote-like histones to wrap their DNA into nucleosomal structures. Furthermore, archaeal DNA displays unique supercoiling flexibility: depending on the species and environment, it can be negatively supercoiled, positively supercoiled (which protects against thermal denaturation in hyperthermophiles), or completely unsupercoiled.

2. Bacterial Plasmids: Classes, Replication, and Copy Number Control

Biophysical Characteristics and Classes

Plasmids (coined by Joshua Lederberg in 1952) are autonomous, self-replicating extrachromosomal dsDNA molecules. Although typically circular and negatively supercoiled, linear dsDNA plasmids exist in genera like Borrelia and Streptomyces. Plasmids generally encode non-essential but highly advantageous traits.

                            Functional Classes of Plasmids

                                    [ Plasmids ]
                                         │
          ┌──────────────┬───────────────┼───────────────┬──────────────┐
          ▼              ▼               ▼               ▼              ▼
    [ Fertility ]  [ Resistance ]  [ Colicinogenic ] [Degradative ] [ Virulence ]
     F-plasmid      R-plasmids       Col-plasmids      TOL plasmid   Ti / Pathogenic
    Conjugation    Drug defenses    Bacteriocins     Unusual carbon   Host invasion
  

• Fertility (F) Plasmids: Large episomes (approximately 100 kb) that direct their own transfer between bacteria via conjugation.
• Resistance (R) Plasmids: Transferable elements encoding enzymes that destroy or modify antibiotics (e.g., beta-lactamases, acetyltransferases).
• Col (Colicinogenic) Plasmids: Synthesise antimicrobial peptides called bacteriocins (such as colicins in E. coli) that kill susceptible, closely related bacterial strains.
• Degradative Plasmids: Encode complete metabolic pathways allowing the host to utilise unusual carbon sources (e.g., toluene via the TOL plasmid, salicylic acid, or camphor).
• Virulence Plasmids: Encode toxins, adhesins, or iron-acquisition systems that convert benign bacteria into pathogens (e.g., the pYV plasmid of Yersinia enterocolitica).
• Cryptic Plasmids: Abundant in nature, these plasmids replicate stably but encode no identifiable phenotypic traits or physiological advantages.

Plasmid Mobility Categories

Based on their transferability, plasmids are classified into three functional groups:

• Conjugative Plasmids: Self-transmissible elements containing two essential gene suites:
  • Mpf (Mating Pair Formation) Genes: Encode the physical structures required for cell-to-cell contact, including the sex pilus and the Type IV Secretion System (T4SS) conjugation bridge.
  • Dtr (Donor Transfer Replication) Genes: Encode proteins involved in processing plasmid DNA for transfer, including the relaxosome and the enzyme relaxase, which nicks the plasmid at the origin of transfer ($oriT$) and covalently attaches to the 5′ end.
• Mobilizable Plasmids: Possess their own $oriT$ and Dtr machinery (such as relaxase) but lack Mpf genes. They cannot initiate transfer independently; however, if a co-resident conjugative plasmid establishes a T4SS bridge, the mobilizable plasmid can “hijack” the bridge and transfer itself.
• Non-mobilizable Plasmids: Lack both transfer replication genes and mating pair formation machinery, remaining restricted to the host cell.

Plasmid Replication Mechanisms

• Theta Replication: The parental dsDNA strands are unwound at a vegetative origin ($oriV$), forming a replication bubble with one or two active replication forks. This bidirectional or unidirectional process creates an intermediate structure resembling the Greek letter theta.
• Rolling Circle (RC) Replication: Initiated by a plasmid-encoded initiator protein (Rep) that introduces a site-specific single-stranded nick at the double-strand origin ($dso$). The 3′-OH end acts as a primer for DNA Polymerase III, which synthesizes a new strand while displacing the parental single strand. The displaced strand is subsequently converted into dsDNA starting from the single-strand origin ($sso$).

Copy Number Control and Regulation

The average number of plasmid molecules per cell immediately after division is its copy number. Plasmids are divided into stringent plasmids (low copy number, typically 1–2 copies per cell) and relaxed plasmids (high copy number, 10–100+ copies per cell). Three primary molecular mechanisms prevent over-replication:

                           Plasmid Regulation Pathways

     1. Antisense RNA Control:
        5' [ antisense RNA ] ──┐ (Binds and sterically blocks)
                               ▼
        5' [ Rep mRNA Primer ] ───X──> No translation of initiator protein

     2. Iteron Binding Control:
        RepA Protein ──┐ (Binds and physically cross-links)
                       ▼
        [ Iteron Repeat ]=====[ Iteron Repeat ] ───X──> Replication blocked (Handcuffing)
  

• Antisense RNA: Small, plasmid-encoded, non-coding RNAs bind to complementary sequences on the replication primer or the initiator protein mRNA, sterically blocking translation or primer processing (e.g., the RNAI/RNAII system in ColE1).
• Iterons: Repetitive 18–22 bp sequences located near $oriV$. As plasmid concentration rises, the initiator protein (e.g., RepA) binds to these iterons and physically cross-links adjacent plasmids (“handcuffing”), rendering the origin inaccessible for replication.
• ctRNA (Counter-transcribed RNA): Non-coding RNAs that pair with a target mRNA to modulate its folding or stability, preventing the synthesis of essential replication proteins.

Plasmid Incompatibility and Partitioning

• Incompatibility (Inc Group): If two different plasmids share identical replication control or partitioning machinery, they cannot stably coexist in the same cell. Because they are regulated as a single unit, cell division randomly segregates them, leading to the rapid loss of one plasmid. These plasmids are said to belong to the same incompatibility group.
• Active Partitioning (Par Systems): To prevent plasmid-free cells during division, low-copy-number plasmids use active partitioning systems (such as the parS-ParA-ParB complex):
  • parS: A cis-acting, centromere-like DNA sequence on the plasmid.
  • ParB: A DNA-binding protein that specifically coats the parS site.
  • ParA: An ATPase that forms dynamic protein filaments. ParA-ATP interacts with the ParB-plasmid complex, and ATP hydrolysis pulls or pushes the sister plasmids to opposite poles of the dividing cell.

Plasmids in Eukaryotes: The 2-micron Circle

Plasmids are not restricted to prokaryotes. The budding yeast Saccharomyces cerevisiae contains the 2-micron circle, a 6.3 kb circular dsDNA plasmid located in the nucleus. Maintained at 50–100 copies per haploid cell, it is coated with eukaryotic histones, organized into nucleosomes, and replicates exactly once per cell cycle using host replication enzymes.

3. Bacterial Nutrition and Culture Media

Macronutrients vs. Micronutrients

• Macronutrients: Required in relatively large quantities. Carbon (C), Oxygen (O), Hydrogen (H), Nitrogen (N), Sulfur (S), and Phosphorus (P) are the structural building blocks of macromolecules. Potassium (K), Calcium (Ca), Magnesium (Mg), and Iron (Fe) function as essential cofactors and osmoregulators.
• Micronutrients (Trace Elements): Required in trace amounts. Manganese (Mn), Zinc (Zn), Cobalt (Co), Molybdenum (Mo), Nickel (Ni), and Copper (Cu) typically serve as active-site catalysts for metal-dependent enzymes.

Nutritional Classification Matrix

Bacteria are classified into four major nutritional groups based on how they satisfy their requirements for Carbon, Energy, and Hydrogen/Electrons:

                               Bacterial Nutrition Matrix

                                    [ ENERGY SOURCE ]
                                     /             \
                        [ Phototrophs ]           [ Chemotrophs ]
                           (Light)                   (Chemicals)
                           /     \                     /       \
               [ Lithotroph ]  [ Organotroph ]  [ Lithotroph ] [ Organotroph ]
                (Inorganic)     (Organic)        (Inorganic)    (Organic)
                    │               │                │              │
                    ▼               ▼                ▼              ▼
               Photolitho-     Photoorgano-     Chemolitho-    Chemoorgano-
               autotrophy      heterotrophy     autotrophy     heterotrophy
  

• Photolithotrophic Autotrophy (Photolithoautotrophy): Energy from light, electrons from inorganic sources, and carbon from CO₂. Representatives: Cyanobacteria, Purple sulfur bacteria.
• Photoorganotrophic Heterotrophy (Photoorganoheterotrophy): Energy from light, electrons and carbon from organic sources. Representatives: Purple and Green non-sulfur bacteria.
• Chemolithotrophic Autotrophy (Chemolithoautotrophy): Energy from oxidizing inorganic compounds, electrons from inorganic sources, carbon from CO₂. Key pathways:
  • Ammonia Oxidisers: $\text{NH}_3 + \text{O}_2 \longrightarrow \text{NO}_2^-$
  • Nitrite Oxidisers: $\text{NO}_2^- + \text{O}_2 \longrightarrow \text{NO}_3^-$
  • Sulfur Oxidisers: $\text{S}^0, \text{S}_2\text{O}_3^{2-} + \text{O}_2 \longrightarrow \text{SO}_4^{2-}$
  • Iron Bacteria: $\text{Fe}^{2+} + \text{O}_2 \longrightarrow \text{Fe}^{3+}$
  • Hydrogen Producers: $\text{H}_2 + \text{O}_2 \longrightarrow \text{H}_2\text{O}$
  • Carboxidobacteria: $\text{CO} + \text{O}_2 \longrightarrow \text{CO}_2$
• Chemoorganotrophic Heterotrophy (Chemoorganoheterotrophy): Energy, electrons, and carbon from organic chemical compounds. Representatives: Most non-photosynthetic bacteria, fungi, and all pathogenic prokaryotes.

Functional Classifications of Culture Media

• Minimal and Supplementary Media: Minimal media contain only the absolute minimum nutrients required to support wild-type (prototroph) growth. Nutritional mutants (auxotrophs) require supplementary media.
• Synthetic (Defined) Media: Prepared from pure chemical compounds; exact molecular formulas and concentrations are known.
• Complex Media: Contain rich, chemically undefined ingredients such as yeast extract or beef broth.
• Enriched Media: Base complex media supplemented with nutritious blood, serum, or egg yolk to support fastidious pathogens.
• Selective Media: Supplemented with specific inhibitory substances (like bile salts) that permit desired microbes while suppressing others.
• Differential Media: Formulated to distinguish between different groups of bacteria based on biochemical properties (e.g., MacConkey Agar).

4. Bacterial Cell Division, Growth Kinetics, and Environmental Responses

Binary Fission and the Role of FtsZ

The primary mode of asexual reproduction in bacteria is binary fission. This process begins with bidirectional replication of the circular chromosome starting at the origin. As replication proceeds, the cell elongates, and the sister chromosomes segregate to opposite poles.

                                 Binary Fission & Septation

                 ┌───────────────────────┐
                 │    (==DNA==)  (==DNA==)│   Chromosome segregation
                 └───────────┬───────────┘
                             ▼
                 ┌───────────  ───────────┐
                 │    (==)  ( FtsZ )  (==)│   FtsZ recruits to center
                 └───────────  ───────────┘   and forms the Z-ring
                             ▼
                         ┌───  ───┐
                         │( )  │( )   Septum constriction
                         └───  ───┘
  

A key molecular player in this division is FtsZ, an evolutionary homologue of eukaryotic tubulin. FtsZ monomers GTP-polymerise at the mid-cell division site, assembling into a highly dynamic ring structure (the Z-ring). The Z-ring recruits accessory division proteins (the divisome) to coordinate the synthesis of the new cell wall and drive septal constriction.

Growth Phases in a Closed Batch Culture

                              Bacterial Batch Growth Curve

               Log[Viable Cells]
                      ▲             [Log / Exponential]
                      │                  /-----\ [Stationary]
                      │                 /       \
                      │                /         \
                      │      [Lag] ___/           \ [Death]
                      │  
                      └─────────────────────────────────► Time
  

• Lag Phase: No increase in cell number occurs. This is an active period of adaptation and ATP accumulation.
• Log (Exponential) Phase: Cells divide at their maximum genetic rate. Population increases in a geometric progression ($2^0 \longrightarrow 2^1 \longrightarrow 2^2 \longrightarrow \dots 2^n$). Generation time remains constant.
• Stationary Phase: The total number of viable cells remains constant, as cell division matches cell death. Triggered by exhaustion of nutrients, accumulation of toxic waste, or physical exhaustion of space.
• Death Phase: Nutrient deprivation and toxicity lead to an exponential decline in viable cell count.

Mathematics of Exponential Growth

During exponential growth, the population size at any given time can be modeled by the geometric progression:

Where:
$N_0$ = Initial number of bacterial cells.
$N$ = Final number of bacterial cells after $n$ generations.
$n$ = Number of generations.

Applying base-10 logarithms to solve for $n$:

The generation (doubling) time ($t_{\text{gen}}$) is calculated by dividing the total duration of the exponential phase ($t$) by the number of generations ($n$):

Worked Numerical Problem

Problem: A bacterial culture contains 500 cells/mL in the exponential growth phase at 8:00 AM. Given a generation time of 20 minutes, calculate the total cell density at 11:00 AM.

Step-by-step Solution:

1. Calculate the elapsed time ($t$):
   $$t = 11:00\text{ AM} – 8:00\text{ AM} = 3\text{ hours} = 180\text{ minutes}$$
2. Calculate the number of generations ($n$):
   $$n = \frac{180\text{ minutes}}{20\text{ minutes}} = 9\text{ generations}$$
3. Apply the exponential growth equation:
   $$N = 500 \times 2^9 = 500 \times 512 = 256,000\text{ cells/mL}$$

Final Answer: At 11:00 AM, the total cell density will be 256,000 cells/mL.

Bacterial Classification Based on Environmental Factors

5. Mechanisms of Horizontal Gene Transfer (HGT) and Recombination

                                 Horizontal Gene Transfer (HGT)

          [ TRANSFORMATION ]          [ TRANSDUCTION ]           [ CONJUGATION ]
              Naked DNA                  Viral Vector             Direct Contact
                ~ ~ ~                       O==O                      ┌───┐   ┌───┐
                  ▼                         │  │                      │   ├───┤   │
                ┌───┐                       ▼                         └───┘ T4SS  └───┘
                └───┘                     ┌───┐                       Donor   Recipient
              Recipient                   └───┘
                                        Recipient
  

5.1 Transformation

Transformation is the uptake of naked, extracellular DNA by a competent cell and its integration into the host genome.

• Gram-Positive Transformation (Streptococcus pneumoniae): Utilizes a competence-stimulating peptide (CSP). dsDNA binds to ComEA, is cleaved by EndA to ssDNA, translocated through ComEC, and integrated by RecA.
• Gram-Negative Transformation (Haemophilus influenzae): Takes up dsDNA using transformasomes. Requires a specific 11-bp uptake sequence ($5’\text{-AAGTGCGGTCA-}3’$) to prevent foreign DNA uptake.

5.2 Transduction

Transduction is the transfer of host bacterial genes mediated by a bacteriophage vector.

                           Generalized vs. Specialized Transduction

     GENERALIZED: Lytic Cycle                  SPECIALIZED: Lysogenic Excision Error

       [ Phage Cap ] <--- Packages random       [ Prophage ] (Imprecise excision)
       [  bacterial ]     bacterial DNA fragment      │
       [  fragment  ]     instead of viral DNA        ▼   Transduces ONLY genes directly
                                                          adjacent to integration site
                                                          (e.g., gal or bio in Lambda)
  

• Generalized Transduction: Occurs during the lytic cycle due to a mistake during capsid packaging (phage terminase accidentally cuts host DNA). Any bacterial gene can be transferred.
• Specialized Transduction: Occurs during the transition from lysogenic to lytic cycle. Imprecise excision of a prophage (like phage $\lambda$) mistakenly incorporates flanking host genes (e.g., $gal$ or $bio$). Only adjacent genes are transferred.

5.3 Conjugation

Transfer of genetic material requiring direct physical contact and a conjugative plasmid.

                           F+ x F- and Hfr Conjugation

      F+ x F- Conjugation:
       ( F+ ) ==[pilus]==> ( F- )  ───>  ( F+ ) and ( F+ )  (Only F-plasmid transfers)

      Hfr x F- Conjugation:
       ( Hfr ) ==[pilus]==> ( F- ) ───> ( Hfr ) and ( F- )  (Chromosomal DNA transfers;
       [F integrated]                                        recipient remains F-)
  

• F+ x F- Conjugation: The F-plasmid is transferred. Both cells become F+.
• Hfr x F- Conjugation: The F-plasmid is integrated into the host chromosome. A segment of the chromosome is transferred; the recipient receives alleles but usually remains F- due to interrupted contact.
• F’ x F- Conjugation (Sexduction): An integrated F-plasmid undergoes imprecise excision, taking chromosomal genes with it to form an F’ plasmid.

6. Chromosomal Gene Mapping Protocols in Bacteria

6.1 Mapping by Transduction (Co-transduction)

The frequency of co-transduction is inversely proportional to the physical distance between genes. The relationship is defined by Wu’s Formula:

Where:
$C$ = Co-transduction frequency (fraction between 0 and 1).
$d$ = Physical distance between the two genes (in minutes).
$L$ = Maximum size of the transducing DNA fragment (for phage P1, $L \approx 2\text{ minutes}$).

Worked Problem: Deciphering Bacterial Gene Order

Problem: Determine the order of four bacterial genes ($a, b, c, d$) based on co-transduction frequencies:

• Set 1 (Select $a$): $d$ (89%) > $b$ (31%) > $c$ (3%). Sequence from $a$: $a — d — b — c$
• Set 2 (Select $b$): $c$ (78%) > $d$ (68%) > $a$ (22%). Confirms $b$ is between $c$ and $d$.
• Set 3 (Select $c$): $b$ (69%) > $d$ (43%) > $a$ (0%). Confirms $c$ is closest to $b$.

Final Answer: The linear order of genes is $a — d — b — c$ (or equivalently $c — b — d — a$).

6.2 Mapping by Conjugation (Interrupted Mating)

By disrupting conjugation at timed intervals, one can plot the entry times of specific alleles. Time of entry is directly proportional to the distance from $oriT$.

                        Interrupted Mating Time of Entry Curves

         % Recombinants
              100 ▲                         /--- leu+
                  │                        /
               50 │                /------/------ lac+
                  │               /
                0 └───X──────────X────────X────────► Time (Minutes)
                      0          5       15
  

Reconstructing the Circular Chromosome

Problem 1: Four Hfr strains derived from wild-type ($m^+ q^+ p^+ n^+ r^+ o^+$) have transfer sequences:

• Strain 1: $m^+ \rightarrow q^+ \rightarrow p^+ \rightarrow n^+ \rightarrow r^+ \rightarrow o^+$
• Strain 2: $o^+ \rightarrow r^+ \rightarrow n^+ \rightarrow p^+ \rightarrow q^+ \rightarrow m^+$ (Reverse of 1)
• Strain 3: $n^+ \rightarrow r^+ \rightarrow o^+ \rightarrow m^+ \rightarrow q^+ \rightarrow p^+$
• Strain 4: $q^+ \rightarrow m^+ \rightarrow o^+ \rightarrow r^+ \rightarrow n^+ \rightarrow p^+$

                              Circular Chromosome Map
                                     [ m ]
                                   /       \
                               [ q ]       [ o ]
                                 │           │
                               [ p ]       [ r ]
                                   \       /
                                     [ n ]
  

Final Answer: The circular gene order is $m — q — p — n — r — o$.

Problem 2: Reconstructing Auxotrophic Loci

• Strain 1: $thr \rightarrow lip \rightarrow trp \rightarrow his \rightarrow thy$
• Strain 2: $str \rightarrow thy \rightarrow his \rightarrow trp \rightarrow lip$
• Strain 3: $his \rightarrow thy \rightarrow str \rightarrow ilv \rightarrow thr$
• Strain 4: $thy \rightarrow his \rightarrow trp \rightarrow lip \rightarrow thr$

                      Circular Linkage Map of Auxotrophic Loci

                                     [ thr ]
                                    /       \
                               [ lip ]     [ ilv ]
                                 │           │
                               [ trp ]     [ str ]
                                 │           │
                               [ his ] ───── [ thy ]
  

Final Answer: The order of the loci on the circular chromosome is $thr — lip — trp — his — thy — str — ilv$.