controlling microbial growth - wouguralnl/gural/318microbialcontroli.pdftitle: biology 218...

Controlling Microbial Growth

• Basic principles• Selection of methods• Physical control

methods• Chemical control

methods• Problems

Hospital Control

Famous DoctorsSemmelweis (1800)

and Lister (early 1900)

Aseptic methods for surgery, boiled instruments

Disinfectants for hands/wounds

Nightingale (1800-1900)Sanitary procedures and trainingProcedures for reducing hospital overcrowding

ContaminationMicrobes present - may or may not be growingInanimate objects - fabrics, food, water…Living objects - hands, animals, bugs…

Why Control Microbes?Prevent/control diseasesPreserve food and increase shelf-lifeQuality control during production, research

Modes of Action

Alteration of cell membranes or wallsDamage to proteins or nucleic acids

Selection of methods• Inexpensive• Fast acting• Stable during storage• Site to be treated• Relative susceptibility:

Factors That Influence Growth

Nutrient and oxygen availabilityTemperature, pH, and pressureMoisture content and salt/sugar

solute levels

General Strategies to Control GrowthKill or inhibit microbes using chemicals or drugsBlock entry for microbes or carriersPromote people’s natural defenses

Physical MethodsHeat1. Dry heat or flaming/

burning - many tools, glass2. Boil/steam 5 min kills most

- NOT spores3. Only autoclave - 121°C 20

min - kills ALL4. Pasteurize (60°C, 30 min.)

does NOT = sterilize. 5. Ultrahigh temperatures =

flash heating 140oC for 1-3 seconds

Miscellaneous Physical MethodsCold/Freezing: slows growth - does not always

killSound waves dislodges - does not kill well

RadiationUV sterilizes surfaces; X-rays penetrateDestroy DNAIncreasingly applied to mail and foods

FiltrationFilters made of nylon, porcelain, gauze, sandPlaced in air vents, water valves, masks, etc…Pores/holes 0.45 microns trap most bacteriaPores of 0.20 microns trap most viruses

Methods

Osmotic pressure

• Drying & Dessication• Salting/brining• pH

Chemical Antimicrobials

Ideal ChemicalsEffect happens in a reasonable timeNon-toxic or non-corrosive to materialsWater-soluble, easy to prepare, stable

Antiseptics for Human Tissues

Detergents disrupt cell membranes—PhenolicsAlcohols, hydrogen peroxide, iodine – disrupt proteins

Antibacterial soaps

Disinfectants for Non-Living ThingsFormalin destroys DNAPhenol, bleach - proteins

Gaseous agents

• Ozone• Ethylene oxide

• Use in a chamber: when you cannot use heat or water soluble chemicals

Arguments Against…Antimicrobial resistanceLoss of “good bacteria” that are protectiveLow immune stimulation, chronic diseases

Hospital-Acquired Infections

Nosocomial infections on the rise!

2 million patients per 40 million per year

Number of deaths = 75,000-100,000

Added cost = $4.5 billion; stay = 2-4 weeks

Community-acquired –present at admission

Nosocomial – acquired in the hospital setting

Iatrogenic – acquired from health care provide

Reservoirs – people, objects, food/water

Why?

Indiscriminate use of antimicrobialsDiminished practice of aseptic

proceduresLonger stays - aging, premature

infants, AIDSPatient overcrowding and staff

shortagesPoorly-trained, low-end, temporary

staffGreater use of immune-

suppressantsIndwelling catheters, respirators,

dialysis

Major Kinds of InfectionsGram Positives – blood, boilsGram Negatives – bladderVarious/both - pneumonia70% agents drug-resistant

Health Care Workers Also At RiskBlood/fluid – e.g. Hepatitis, AIDSAerosolized respiratory agents - e.g. InfluenzaContact contagious like Staphylococcus30-70% carry resistant Staphylococcus

Health Care Infection Control Procedures

Medical Asepsis - Clean TechniqueGoals– exclude pathogens, prevent transferHandwashing, handwashing, handwashingProper handling of instruments, fomites, waste

Standard PrecautionsGoal - reduce transmission of via body fluidsApplied to ALL patients, regardless of statusIncludes medical asepsis plus gloves, masks

Surgical Asepsis - Sterile TechniqueGoal One – to exclude ALL microorganismsGoal Two - to keep objects and areas sterileEmploys only sterile tools and techniquesUsed during invasive procedures and in all labs

Hospital Infection Control Committee (ICC)Surveillance, produce reports, educate staffSet policy and take drastic actionCommunicate with local public health Work with hospital clinical microbiology lab