A General Model for Biofilm Development

Biofilm formation is a developmental process in which bacteria undergo a regulated lifestyle switch from a nomadic unicellular state to a sedentary multicellular state where subsequent growth results in structured communities and cellular differentiation. Results of prior work by many groups allow the construction of a hypothetical developmental model for biofilm formation that can be generalized for many different bacterial species. This model can be adjusted to fit either of two general modes of unicellular lifestyle: nonmotile and motile.

Transport of an Infectious Agent

Transmission involves the transport of an infectious agent from the reservoir to the host. It is the most important link in the chain of infection. Pathogens can be transmitted from the reservoir to a susceptible host by various routes (Sobsey and Olson, 1983).

a. Person-to-Person Transmission

The most common route of transmission of infectious agents is from person to person. The best examples of direct contact transmission are the sexually transmitted diseases such as syphilis, gonorrhea, herpes, or acquired immunodeficiency syndrome (AIDS).

Determination of Cell Biochemicals

Microbial biomass can also be measured by determination of specific cell biochemical constituents such as ATP, DNA, RNA, proteins, phospholipids, bacterial cell wall components, or photosynthetic pigments (Sutton, 2002).

a.        ATP

Adenosine triphosphate has often been used to determine live microbial biomass in environmental samples, using a ratio of C/ATP = 250 for aquatic samples. However, the ATP content of cells varies with the growth rate and metabolic state of microorganisms and nutrient limitation. A better measure is the total adenylate pool AT (AT = ATP + ADP + AMP) because it does not change greatly with changes in metabolic activities of the microorganisms. The adenylate energy charge (EC) ratio provides information on growth potential of naturally occurring microbial populations.

Fungi (Eukaryote)

Fungi are eukaryotic organisms that produce long filaments called hyphae, which form a mass called mycellium. Chitin is a characteristic component of the cell wall of hyphae. In most fungi, the hyphae are septate and contain crosswalls that divide the filament into separate cells containing one nucleus each. In some others,the hyphae are nonseptate and contain several nuclei. They are called coenocytic hyphae.

Unusual Types of Bacteria (Part 3)

. Actinomycetes

Actinomycetes are gram-positive filamentous bacteria characterized by mycelial growth (i.e., branching filaments), which is analogous to fungal growth. However, the diameter of the filaments is similar in size to bacteria (approximately 1 mm). Most actinomycetes are strict aerobes, but a few of them require anaerobic conditions. Most of these microorganisms produce spores, and their taxonomy is based on these reproductive structures (e.g., single spores in Micromonospora or chains of spores in Streptomyces). They are commonly found in water, wastewater treatment plants, and soils (with preference for neutral and alkaline soils). Some of them (e.g., Streptomyces) produce a

Unusual Types of Bacteria (Part 2)

. Gliding Bacteria

These filamentous gram-negative bacteria move by gliding, a slow motion on a solid surface. They resemble certain cyanobacteria except that they are colorless. Beggiatoa and Thiothrix are gliding bacteria that oxidize H2S to S0, which accumulates as sulfur granules inside the cells. Thiothrix filaments are characterized by their ability to form rosettes. Myxobacteria are another group of gliding microorganisms. They feed by lysing bacterial, fungal, or algal cells. Vegetative cells aggregate to make “fruiting bodies,” which lead to the formation of resting structures called myxospores. Under favorable conditions, myxospores germinate into vegetative cells.

. Bdellovibrio (B. bacteriovorus)

These small (0.2–0.3 mm), flagellated (polar flagellum) bacteria are predatory on gram-negative bacteria. After attaching to the bacterial prey, Bdellovibrio penetrates the cells and multiplies in the periplasmic space (space between the cell wall and the plasma membrane). Because they lyse their prey, they are able to form plaques on a lawn of the host bacterium. Some Bdellovibrio can grow independently on complex organic media.

Unusual Types of Bacteria (Part 1)

. Sheathed Bacteria

These bacteria are filamentous microorganisms surrounded by a tubelike structure called a sheath. The bacterial cells inside the sheath are gram-negative rods that become flagellated (swarmer cells) when they leave the sheath. The swarmer cells produce a new sheath at a relatively rapid rate. They are often found in polluted streams and in wastewater treatment plants. This group includes three genera: Sphaerotilus, Leptothrix, and Crenothrix. These bacteria have the ability to oxidize reduced

DNA Replication and Protein Synthesis

Replication: The DNA moleculecan make an exact copy of itself. The two strands separate and new complementary strands are formed. The double helix unwinds and each of the DNA strands acts as a template for a new complementary strand. Nucleotides move into the replication fork and align themselves against the complementary bases on the template. The addition of

Cytoplasmic Membrane (Plasma Membrane)

The cytoplasmic membrane is a 40–80 A ˚ -thick semipermeable membrane that contains a phospholipid bilayer with proteins embedded within the bilayer (fluid mosaic model) (Fig. 1.3). The phospholipid bilayer is made of hydrophobic fatty acids oriented towards the inside of the bilayer and hydrophilic glycerol moieties oriented towards the outside of the bilayer. Cations such as

HACCP

There is growing acceptance throughout the EU and in many other countries of the value of HACCP principles in ensuring the microbiological safety of foods. The HACCP approach is a systematic way of analysing the potential hazards of a food operation, identifying the points in the operation where the hazards may occur, and where controls over those that are important to consumer safety can be achieved. Most of the product-specific EC directives as well as the Directive on the Hygiene of Foodstuffs (93/43/EEC), place obligations on industry and food business operators to adopt HACCP principles as the basis for their product safety management systems. The advantages of the HACCP approach over a food safety control system based purely on microbiological standards is now widely recognized. Thus, the Commission proposes to consolidate and simplify existing EC food hygiene legislation. These are expected to be implemented by 2004. The proposed consolidation adopts a unified approach to hygiene and extends the general hygiene rules and HACCP principles to cover hygiene throughout the food chain, including primary production, i.e. the ‘farm-to-fork’ approach to managing food safety. Responsibility of food safety will be unambiguously placed onto food producers. A fully documented HACCP plan will be required of all food producers, including caterers, regardless of size.

This will include a specific monitoring programme, thereby reinforcing the own-check principle of food producers. An absolute requirement for full traceability of all foods and ingredients used in food production is also introduced, such that all food producers must keep adequate records to allow full traceability throughout the products’ allotted shelf-life.

Measurement of Active Cells in Environmental Samples

Several approaches have been considered for assessing microbial viability/activity in environmental samples. Epifluorescence microscopy, in combination with the use of oxido-reduction dyes, is used to determine the percent of active cells in aquatic environments. The most popular oxido-reduction dyes are INT (2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride) and CTC (cyanoditolyl tetrazolium chloride) (Poschet al., 1997; Pyle et al., 1995a). A good correlation was found between