Bacterial Growth and Requirements PDF

Dr/ Ingy Nassar Bacterial growth requirements The main activity of the bacteria is reproduction, this requires the following: I. Nutritional Requirements Nutrients in growth media must contain all elements necessary for the biologic synthesis of new organisms (Basic elemen...

Dr/ Ingy Nassar Bacterial growth requirements The main activity of the bacteria is reproduction, this requires the following: I. Nutritional Requirements Nutrients in growth media must contain all elements necessary for the biologic synthesis of new organisms (Basic elements)= hydrogen, oxygen, carbon and nitrogen in large quantities, with sulphur and phosphorus in smaller amounts, while other elements such as sodium, potassium, iron, magnesium and manganese in traces. II. Growth factors These are essential constituents which cannot be synthesized by the organisms and must be provided readymade. Examples of bacterial growth factors are: a) Amino acids b) Bacterial vitamins. II. Energy The mechanisms for generating metabolic energy include aerobic respiration, anaerobic respiration, and fermentation. Environmental Factors Required for Bacterial Growth 1. Hydrogen ion concentration 2. Temperature: Almost all bacteria which are pathogenic to man have an optimum temperature for growth at 37°C (i.e. body temperature). 3. Aeration: Two gases influence the growth of bacteria; oxygen and carbon dioxide. As regards oxygen, the bacteria can be classified into a) Obligatory aerobes: These can grow only in the presence of oxygen, e.g. Mycobacterium tuberculosis and Pseudomonas aeruginosa. b) Facultative anaerobes: These organisms are able to grow in the presence or absence of oxygen e.g. Enterobacteria and Staphylococci. c) Obligatory anaerobes: These organisms cannot grow in the presence of oxygen, e.g. Clostridia, some Spirochetes and anaerobic Streptococci. d) Micro-aerophilics: These organisms grow best in the presence of little oxygen. Little or no growth occurs in the presence of free oxygen. They usually prefer a relatively high concentration of CO2, e.g. Campylobacter and Helicobacter. Environmental Factors Required for Bacterial Growth 4. Ionic strength and osmotic pressure 5. Moisture and desiccation: Moisture is absolutely necessary for growth. Drying in air is lethal to many microbes BACTERIAL GROWTH CURVE When a comparatively small number of organisms are taken from a culture and inoculated into a fresh growth medium, the number of cells multiply a million-fold or more. BACTERIAL GROWTH CURVE If the logarithmic number of cells present at different times after incubation is measured and plotted in relation to the time, the resultant plot is the growth curve BACTERIAL GROWTH CURVE Four main phases can be recognized: 1. Lag phase There is no multiplication of bacteria or even depletion. It represents the time taken for the cells to adapt themselves in the new environment and prepare for division. The duration of this phase varies from few hours to few days. 2. Logarithmic phase (Log):(or phase of accelerated growth) The cell divides by binary fission regularly at a constant maximal rate and in an exponential manner, so that there is a linear relationship between the log of the number of cells and time. In this phase the bacteria are more sensitive to the effect of antimicrobial agents. 3. Stationary phase - Exhaustion of nutrients. - Accumulation of toxic metabolites. - Development of unfavorable pH and O2 consumption The rate of growth diminishes until the number of cells remains constant, where the number of newly formed cells becomes equal to that dying. 4. Decline or death phase During this phase, the bacterial population declines due to death of the cells. The decline phase starts due to: 1. accumulation of toxic products and autolytic enzymes. 2. exhaustion of nutrients.  MCQ 1. Organisms which grow only in absence of oxygen are termed a. Obligatory Aerobes b. Obligatory Anaerobes c. Facultative anaerobes d. Micro-aerophilic 2. Organisms which grow better in low oxygen concentrations are termed a. Obligatory Aerobes b. Obligatory Anaerobes c. Facultative anaerobes d. Micro-aerophilic 3. Organisms which can grow well in presence or absence of oxygen are termed a. Obligatory Aerobes b. Obligatory Anaerobes c. Facultative anaerobes d. Micro-aerophilic  MCQ 1. Organisms which grow only in absence of oxygen are termed a. Obligatory Aerobes b. Obligatory Anaerobes c. Facultative anaerobes d. Micro-aerophilic 2. Organisms which grow better in low oxygen concentrations are termed a. Obligatory Aerobes b. Obligatory Anaerobes c. Facultative anaerobes d. Micro-aerophilic 3. Organisms which can grow well in presence or absence of oxygen are termed a. Obligatory Aerobes b. Obligatory Anaerobes c. Facultative anaerobes d. Micro-aerophilic 4. --------- in the growth curve of bacterial culture is reached when number of new cells becomes equal to dying cells a. Lag phase b. Log phase c. Stationary phase d. Decline phase 5 ---------- in the growth curve of bacterial culture is reached when there is accumulation of toxic products and autolytic enzymes a. Lag phase b. Log phase c. Stationary phase d. Decline phase 6. In the ------------ in the growth curve of bacterial culture, Bacteria don’t divide in e and take time to adapt to the new environment a. Lag phase b. Log phase c. Stationary phase d. Decline phase 7. In the ----------------- in the growth curve of bacterial culture, bacteria undergo cell division at maximal constant rate a. Lag phase b. Log phase c. Stationary phase 4. --------- in the growth curve of bacterial culture is reached when number of new cells becomes equal to dying cells a. Lag phase b. Log phase c. Stationary phase d. Decline phase 5 ---------- in the growth curve of bacterial culture is reached when there is accumulation of toxic products and autolytic enzymes a. Lag phase b. Log phase c. Stationary phase d. Decline phase 6. In the ------------ in the growth curve of bacterial culture, Bacteria don’t divide in e and take time to adapt to the new environment a. Lag phase b. Log phase c. Stationary phase d. Decline phase 7. In the ----------------- in the growth curve of bacterial culture, bacteria undergo cell division at maximal constant rate a. Lag phase b. Log phase c. Stationary phase Dr/ Ingy Nassar The normal microbial flora These organisms live in complete harmony with the host without causing any damage to it. In a healthy person (with intact immunological and anatomical defenses) such organisms rarely cause disease, only under certain conditions when a suitable opportunity arises= opportunists, opportunistic pathogens. The normal microbial flora The normal flora may be divided into 2 groups: 1) Resident: It consists of organisms, which are regularly present in a given area at a given age and when disturbed it soon re-establishes itself, e.g.  Escherichia coli in the intestine  Staphylococcus epidermidis on skin  Alpha-haemolytic Streptococci on oropharynx 2) Transient: It consists of both pathogens and non- pathogens that inhabit the skin or mucous membrane for a limited period (from seconds to weeks). If the resident flora is intact, there is very little significance of the transient flora. However, if the normal resident flora is disturbed, the transient flora may colonize, proliferate, and produce disease.  The normal microbial flora Beneficial role of resident flora: A- They prevent or suppress the colonization or invasion of the body by pathogens by bacterial interference. B- Members of the intestinal flora synthesize some vitamins, especially vitamin K and several B vitamins, others aid in the digestion and absorption of nutrients. The normal microbial flora Normal flora at various sites of the body: Skin and mucous membrane always carry a variety of organisms because they are in contact with the environment. All internal organs (except GIT) and body fluids are normally sterile (free from any organisms). Members of Normal Flora Anatomic location Bacteroides species Colon Clostridium species Colon Corynebacterium species Nasopharynx, skin, vagina (diphtheroids) Enterococcus faecalis Colon E. Coli Colon Haemophilus species Nasopharynx, conjunctiva Lactobacillus species Mouth, colon, vagina Staphylococcus aureus Nose Staphylococcus epidermidis Skin, nose, mouth, vagina Viridans streptococci Mouth, nasopharynx  Pathogenesis of bacterial infection Colonization occurs when microorganisms live on or in a host organism but do not invade tissues or cause damage. Infection is the invasion of the host by microorganisms, which then multiply in close association with the host’s tissues. Bacterial infectivity results from a disturbance in the balance between bacterial virulence and host resistance. The human body carries 10 times more bacteria than human cells and only few bacterial species we carry cause disease. Types of Microorganisms 1. Commensal or normal microbial flora Consist of those micro-organisms, which are present on body surfaces covered by epithelial cells and are exposed to the external environment (gastrointestinal and respiratory tract, vagina, skin, etc.). 2. Opportunistic microorganisms (potential pathogens) These potential pathogens can cause diseases under certain conditions; a) When host defense mechanism is lowered as in case of diabetics, immunosuppressed or leukemic patients. b) When the natural habitat of the microorganism is changed. e.g. E.coli leaves the intestine causing urinary tract infection. c) When alteration in the host tissue occurs. e.g. viridans streptococci (one of the normal oral flora) may produce subacute bacterial endocarditis in a previously damaged heart valve, if the organism reaches the blood stream after tooth extraction or tonsillectomy. 3. Pathogenic microorganisms (True Pathogens): Pathogens capable of infecting the tissues of normal healthy subjects. Properties of True Pathogens: a. Infectivity: - The pathogen must have a portal of entry to the host in order to initiate infection. - The capacity to initiate infection is related to dose of the pathogen (the infective dose) b. Virulence: Degree of pathogenicity of the organism Virulence factors: These are certain structures or products that help the microorganisms to: (1) Invade the host, (2) Cause disease, and (3) Evade host defenses. c. Transmissibility: Organisms should have 1. Portal of exit to shed off in large amount. 2. There must be a mode of transmission (contact; direct, indirect, droplet, airborne, vehicle, vector- borne) to a new host  Types of Microorganisms Virulence factors: i. Adherence factors e.g Pili. ii. Invasiveness. iii. Antiphagocytic factors e.g Capsule. iv. Toxin production. v. Extracellular enzymes.