Tutor profile: Ray H.

Why are most community-acquired pneumonias due to gram-positive bacteria, while most urinary tract infections are due to gram-negative bacteria?

Gram-positive bacteria primarily live on the skin in environments that are open to air, while gram-negative bacteria prefer less oxygen-rich environments such as the colon. As you move down the digestive tract, the microbiome begins to shift from predominantly positive to predominantly negative. Most pneumonia-causing pathogens enter the body through the mouth, which is likely to contain Staphylococcus, via inhalation of droplets. Urinary tract infections, however, are more likely to be caused by bacteria originating from the anus, primarily E. coli.

What constants are needed in order to calculate the number of molecules in 3.0 grams of table salt (NaCl)?

The Avogadro Constant (6.022x10^23), the molar mass of sodium, & the molar mass of chlorine. Alternately, if the molar mass of sodium chloride is known, it does not need to be calculated.

If the incidence of cystic fibrosis - an autosomal recessive disorder - is 1 in 2,000 live births, how many heterozygous carriers would we expect to see in a population of 2,000?

By using the Hardy-Weinburg equation, where p^2 + 2pq + q^2 = 1 and p + q =1, we can find that: q = 1/2000 = 0.0005 p + 0.0005 = 1 p = 0.9995 So that the incidence of heterozygotes with one non-affected allele p and one affected allele q is 2pq from p^2 + 2pq + q^2 = 1. 2(0.9995)(0.0005) = 0.0009995 If we were to multiply by 100, we could see that 0.09995% (rounded to 0.1%) of the population carries the cystic fibrosis gene. However, the question asked how many carriers we would expect to see in a population of 2,000, so we need to calculate how many people are in 0.1% of 2,000. Since 0.1 * 2,000 = 200, 200 people in a group of 2,000 carry the cystic fibrosis gene but are not affected by it.