(B) LactobacillusoMost micro organisms prefer a neutral or slightly alkaline growth medium pH 7-7.4.oSome microorganisms like acidic or alkaline environments:-Acidophiles: Microorganisms that grow best in acidic media pH 2-5. Eg., Fungi & Helicobacter pylori.-Alkallphiles: Microorganisms that grow best in alkaline media pH. 8.5-11. Eg., Vibrio cholera (the only alkaliphilic human pathogen).-Neutrophiles: Bacteria that can live an multiply within the range of pH 5-8. Most bacteria are neutrophiles.[?]pH:oMost medically important bacteria grow at neutral or slightly alkaline pH (7.2 to 7.6).oVery few bacteria grow below pH 4.oLactobacilli grow in acidic pH; cholera vibrio grow in alkaline pH.oGrowth media includes chemical buffers to prevent acid production.oLactobacillus bacteria, which are an important part of the normal microbiota of the vagina, can tolerate acidic environments at pH values 3.5-6.8 and also contribute to the acidity of the vagina (pH of 4, except at the onset of menstruation) through their metabolic production of lactic acid.oVagina's acidity plays an important role in inhibiting other microbes that are less tolerant of acidity.oFoods are preserved by acids produced by bacterial fermentation.oGrowth is poor below pH 6.0 & above 7.8. Growth stops below pH 5.0 & above 9.0oLactobacillus species grows at acidic pH & known as acidophiles. Vibrios are sensitive to acid but tolerate alkali.[?]Physical growth factors:o1-Acidity (pH requirements):-Pathogenic bacteria grow best at neutral or biological pH which is typically between pH 6.8 to pH 7.4.-Fungi such as yeasts & molds prefer slightly more acidic conditions and grow best between pH 5 to pH 6.-The pathogen Helicobacter pylori is able to survive with pH of the stomach by producing urease. So it is acidophilic bacterium.-On the other extreme, bacteria that prefer alkaline (basic) conditions are known as alkaliphiles. Ex., Vibrio cholerae (prefers pH 9).[?]Acidophiles or acidophilic organisms are those that thrive under highly acidic conditions (at pH 2.0 or below).oThese organisms can be found in different branches of the tree of life, including Archaea, Bacteria, and Eukarya.-Acidobacterium, a phylum of Bacteria-Acidithiobacillales, an order of Proteobacteria e.g. A. ferrooxidans, A. thiooxidans-Thiobacillus prosperus, T. acidophilus, T. organovorus, T. cuprinus-Acetobacter aceti, a bacterium that produces acetic acid (vinegar) from the oxidation of ethanol.-Alicyciobacillus, a genus of bacteria that can contaminate fruit juices.-Helicobacter pylori, a bacterium found in the human stomachOther Options[?]Neutrophile bacteria:-They grow optimally at a pH within one or two pH units of the neutral pH of 7.-Most familiar bacteria, like Escherichia coli, staphylococci, and Salmonella spp. are neutrophiles and do not fare well in the acidic pH of the stomach.-However, there are pathogenic strains of E coli, S. typhi, and other species of intestinal pathogens that are much more resistant to stomach acid. In comparison, fungi thrive at slightly acidic pH values of 5.0-6.0.[?]Alkaiiphile Bacteria:-At the other end of the spectrum are alkaliphiles, microorganisms that grow best at pH between 8.0 and 10.5. Vibrio cholerae, the pathogenic agent of cholera, grows best at the slightly basic pH of 8.0; it can survive pH values of 11.0 but is inactivated by the acid of the stomach.-When it comes to survival at high pH, the bright pink archaean Natronobacterium, found in the soda lakes of the African Rift Valley, may hold the record at a pH of 10.5.-Extreme alkaliphiles have adapted to their harsh environment through evolutionary modification of lipid and protein structure and compensatory mechanisms to maintain the proton motive force in an alkaline environment.-E.g., the alkaiiphile Bacillus firmus derives the energy for transport reactions and motility from a Na+ ion gradient rather than a proton motive force.-Many enzymes from alkaliphiles have a higher isoelectric point, due to an increase in the number of basic amino acids, than homologous enzymes from neutrophiles.