Chapter 1 : Introduction

A typical helminth life cycle (example: Enterobius vermicularis).

Typical fungal structures. On the left, filamentous fungus (molds). Hyphae are shown as long lines of unseparated cells, with the development of a fruiting body with attached spores. On the right, typical unicellular fungal (yeast) cells. Cells are generally polymorphic. In two cases, budding cells are shown. Image on right courtesy of CDC-PHIL/Dr. Libero Ajello, 1972 (ID#4219), with permission.

Simplified fungal cell envelope. The cross-linked cell wall is associated with the inner cell membrane. The cell wall usually consists of the innermost fibrils of chitin or cellulose, with outer layers of amorphous, cross-linked glucans.

Life cycle of Toxoplasma gondii.

Simple representation of a mycoplasma cell surface structure.

Basic structure of a bacterial cell, showing the cell surface in greater detail.

Bacterial cell wall structures. The cell membrane is a similar structure in all types. Gram-positive bacteria have a large peptidoglycan layer (shown as crossed lines) with associated polysaccharides and proteins. Gram-negative bacteria have a smaller peptidoglycan layer linked to an outer membrane. The mycobacterial cell has a series of covalently linked layers, including the peptidoglycan-, arabinogalactan- and mycolic acid-containing components.

Basic structure of peptidoglycan. Polysaccharides of repeating sugars are cross-linked by peptide bridges. Two different types of peptide bridges, which have been described in Gram-positive and Gram-negative bacterial cell walls, are shown.

Cells of M. tuberculosis. Courtesy of Clifton Barry, NIAID.

Basic viral structures.

Typical viral life cycle. Stages include (1) attachment, (2) penetration into the cell, and (3) multiplication. Depending on the virus type, viral particles can be released by cell lysis (4a) or by budding (4b); alternatively, the virus can remain dormant in the cell (4c).

E. coli bacteriophages. The T phages are complex DNA viruses; MS2 and ɸ6 are RNA viruses, with ɸ6 being enveloped.

Theory of prions as infectious proteins. PrPc is the normal form of the protein, and PrPres is the abnormal form.

A representation of the proposed secondary structure changes in the two forms of the prion protein, PrP.

The general structure of LPS, a bacterial endotoxin.

Typical fungal aflatoxin structure.

General microbial resistance to biocides and biocidal processes. This classification can vary depending on the biocide or biocidal process under consideration.

A typical time-kill or D-value determination. A known concentration of the test culture is exposed to the biocide (in this case a chemical solution), samples are withdrawn at various times, they are neutralized, and the population of survivors is determined by incubation on growth media. The actual exposure can be conducted at various temperatures, in the presence/absence of test soils, or other test conditions.

Determination of the D value on microbial exposure to a biocide.

Typical survivor curves on biocide exposure. Curve 1 is concave downward, curve 2 is sigmoidal, and curve 3 is concave upward.

Qualitative and semiquantitative population determination.

D-value estimation using most probable number estimations.

Example of biological indicators, including (from left to right) simple inoculated thread, coupons and wires, paper in sealed pouches (center), and two examples of self-contained biological indicators (with a rapid-read example on the far right). Far right image courtesy of 3M Health Care, with permission.

Example of a chemical indicator color change.

The rate of microbial inactivation on exposure to sterilization processes. In this case, the test microorganism (generally bacterial spores) at a starting population of 106 is exposed to the sterilizing agent under two conditions (A and B). The number of microorganisms can be determined over the contact time with or dose of the antimicrobial using a combination of direct enumeration and fraction negative methods (solid lines). Note that the microbial population (y axis) is plotted in a log10 scale. In process A, “tailing” is observed which may not allow the extrapolation of the kill curve to a defined probability of survival (known as a sterility assurance level, SAL). In process B, the kill curve is linear, allowing extrapolation (dotted line) to an SAL of 10−6.

Basic structures of surfactants/soaps and micelles (a water-in-oil micelle is shown).

Examples of single-chamber (left) and multiple-chamber (right) washer-disinfector machines. Washer-disinfectors come in a variety of shapes and sizes depending on their required use. Images courtesy of STERIS, and ©2017 Getinge AB, with permission.

Various types of cleaning chemistries. Image courtesy of STERIS, with permission.