- Bacteria
What is Bacteria
Humans' relationship with bacteria is complex. Sometimes bacteria lend us a helping hand, such as by curdling milk into yogurt or helping with our digestion. In other cases, bacteria are destructive, causing diseases like pneumonia and methicillin-resistant Staphylococcus aureus (MRSA).
Structure
Bacteria (singular: bacterium) are classified as prokaryotes, which are single-celled organisms with a simple internal structure that lacks a nucleus, and contains DNA that either floats freely in a twisted, thread-like mass called the nucleoid, or in separate, circular pieces called plasmids. Ribosomes are the spherical units in the bacterial cell where proteins are assembled from individual amino acids using the information encoded in ribosomal RNA.
A few different criteria are used to classify bacteria. The organisms can be distinguished by the nature of their cell walls, by their shape, or by differences in their genetic makeup.
The Gram stain is a test used to identify bacteria by the composition of their cell walls, named for Hans Christian Gram, who developed the technique in 1884. The test stains Gram-positive bacteria, or bacteria that do not have an outer membrane. Gram-negative bacteria don't pick up the stain. For example, Streptococcus pneumoniae (S. pneumoniae), which causes pneumonia, is a Gram-positive bacterium, but Escherichia coli (E. coli) and Vibrio cholerae, which causes cholera, are Gram-negative bacteria.
There are three basic bacterial shapes: Round bacteria called cocci (singular: coccus), cylindrical, capsule-shaped ones known as bacilli (singular: bacillus); and spiral bacteria, aptly called spirilla (singular: spirillum). The shapes and configurations of bacteria are often reflected in their names. For example, the milk-curdling Lactobacillus acidophilus are bacilli, and pneumonia-causing S. pneumoniae are a chain of cocci. Some bacteria take other shapes, such as stalked, square or star.
2. Culture Media
Culture Media Preparation
Culture media, also known as growth media, are specific mixtures of nutrients and other substances that support the growth of microorganisms such as bacteria and fungi (yeasts and molds). Culture media are used for quality control tests of nonsterile raw materials and finished products as well as for microbial contamination (sterility) tests in applications such as hygiene monitoring, sterilization process validation and determination of the effectiveness of preservatives and antimicrobial agents.
Microbial enumeration tests for nonsterile products are performed according to the harmonized test procedures of the European, US and Japanese pharmacopoeias. These tests, formerly known as Microbial Limit Tests (MLT), determine the bioburden of the product sample. This is achieved by counting the number of colony forming units that have grown on the culture media for a known sample size (CFU/g). Acceptance criteria are based on the Total Aerobic Microbial Count (TAMC), the Total Yeast and Mold Count (TYMC). Furthermore, specific tests are undertaken to confirm the presence or absence of certain microorganisms that may prove to be objectionable.
The culture media ingredients vary according to the test being performed and the microorganism of interest. Most commonly, culture media are based on a nutrient broth (liquid), often mixed with agar and prepared in petri dishes (they may be semi-solid or solid). For enriched or selective media, further specific components are added.
Culture media preparation must be carried out accurately to ensure microbiological growth is correctly promoted. The individual ingredients of the culture media (powders, gels and liquids) must be carefully weighed out according to the culture media formulation recipe. A precision balance with readability from 1 mg up to 10 mg is typically used for the main components. An analytical balance may be required for weighing out trace elements such as copper and zinc. If a smaller amount of culture media is required, care must be taken to correctly recalculate the ingredient quantities and a balance with a higher readability may be required in order to fulfil accuracy requirements.