micro

• Microbes can be divided into cellular (bacteria, archaea, algae, protozoa, and fungi) and acellular (viruses, viroids, and prions)
• Cellular microbes can be prokaryotic (bacteria and archaea) or eukaryotic (algae, protozoa, and fungi)
• Viruses, viroids, and prions are acellular microbes or infectious particles
• Viruses infect humans, animals, plants, fungi, protozoa, algae, and bacterial cells
• Some viruses cause specific types of cancer, known as oncogenic viruses or oncoviruses
• A typical virion consists of a genome of either DNA or RNA, surrounded by a capsid composed of protein units called capsomeres
• Viruses are classified by type of genetic material, shape and size of capsid, number of capsomeres, presence or absence of an envelope, type of host it infects, disease it produces, target cell(s), and immunologic/antigenic properties
• Steps in the multiplication of animal viruses are attachment, penetration, uncoating, biosynthesis, and assembly
• Animal viruses escape from host cells by lysis or budding, becoming enveloped viruses
• Latent virus infections allow the virus to hide from the host's immune system by entering cells and remaining dormant
• Oncogenic viruses cause cancer, examples include Epstein-Barr virus, human papillomaviruses, and HIV
• Antibiotics are not effective against viral infections, antiviral agents are used to treat viral infections
• Bacteriophages are viruses that infect bacteria, categorized as virulent or temperate bacteriophages
• The lytic cycle of bacteriophages includes attachment, penetration, biosynthesis, assembly, and release
• Viroids are short, naked fragments of single-stranded RNA that interfere with plant cell metabolism
• Prions are small infectious proteins that cause fatal neurologic diseases in animals and humans
• Bacteria are divided into Gram-negative with a cell wall, Gram-positive with a cell wall, and those lacking a cell wall (Mycoplasma spp.)
• Characteristics used in bacterial classification and identification include cell morphology, staining reactions, motility, colony morphology, and biochemical activities
• Three basic categories of bacteria based on shape are cocci (round), bacilli (rod-shaped), and curved/spiral-shaped bacteria
• Cocci may be seen singly, in pairs, chains, clusters, tetrads, or octads
• Bacilli may occur singly, in pairs, chains, filaments, or branched
• Examples of medically important bacilli include Escherichia, Klebsiella, Proteus, Pseudomonas, Haemophilus, and Bacillus spp.
• Examples of curved bacteria include Vibrio spp., Campylobacter spp., and Helicobacter spp.
• Examples of spiral-shaped bacteria include Treponema spp. and Borrelia spp.
• Three major categories of staining procedures for bacteria are simple staining procedure, structural staining procedures like capsule staining, spores staining, and flagella staining, and differential staining procedures like gram and acid-fast staining procedure
• Three major categories of staining procedures:

  • Simple staining procedure
  • Structural staining procedures:
  • Differential staining procedures
• Bacterial smears must be fixed prior to staining
• Two most common techniques of fixation:

  • Heat fixation: excess heat will distort bacterial morphology
  • Methanol fixation: a standardized technique and the preferred method
• Gram Staining Procedure divides bacteria into two major groups:

  • Gram-positive (blue to purple)
  • Gram-negative (pink to red)
• Gram-positive bacteria have a thick layer of peptidoglycan, making it difficult to remove the crystal violet - iodine complex

  • Gram-negative organisms have a thin layer of peptidoglycan, making it easier to remove the crystal violet; the cells are subsequently stained with safranin
• Some bacteria are neither consistently purple nor pink after Gram staining; they are known as Gram-variable bacteria

  • Mycobacterium spp. are often identified using the acid-fast stain
• Acid-fast stain:

  • Carbol fuchsin is the red dye that is driven through the bacterial cell wall using heat
  • Heat is used to soften the waxes in the cell wall
  • Mycobacteria are not decolorized by the acid-alcohol mixture, they are said to be acid-fast
• If a bacterium is able to “swim,” it is said to be motile

  • Bacterial motility is most often associated with flagella and less often with axial filaments
• Colony morphology includes size, color, overall shape, elevation, and the appearance of the edge or margin of the colony

  • Colony morphology is an important clue for identification of bacteria
• Bacteria can be classified based on their atmospheric requirements, including their relationship to O2 and CO2

  • Capnophilic organisms grow best in the presence of increased concentrations of CO2
• All bacteria require elements such as carbon, hydrogen, oxygen, sulfur, phosphorus, and nitrogen for growth

  • Some bacteria require special elements like calcium, iron, or zinc
  • Organisms with demanding nutritional requirements are said to be fastidious (fussy)
• As bacteria grow, they produce waste products and secretions, some of which are enzymes

  • Pathogenic strains of bacteria can be tentatively identified by the enzymes they secrete
  • To identify bacteria in the laboratory, they are inoculated into various substrates to determine their enzymatic activity
• Many pathogens cause disease because they possess capsules, fimbriae, endotoxins, or because they secrete exotoxins and exoenzymes

  • Pathogenicity is often tested by injecting the organism into mice or cell cultures
• Laboratory identification of bacteria is moving towards analyzing the organism’s DNA or RNA through molecular diagnostic procedures

  • DNA probes make it possible to identify an isolate without relying on phenotypic characteristics
• Rickettsias, chlamydias, and mycoplasmas are bacteria that do not possess all the attributes of typical bacterial cells

  • Mycoplasmas are the smallest of the cellular microbes and lack a cell wall
  • Mycoplasmas cause primary atypical pneumonia and genitourinary infections