**Core Concept**
Koch's postulates are a set of criteria used to establish a causal relationship between a microorganism and a disease. These postulates were originally proposed by Robert Koch in 1890 and are still widely used today in microbiology and medicine. The four postulates are: (1) the microorganism must be found in all organisms suffering from the disease, but not in healthy organisms, (2) the microorganism must be isolated from the diseased organism and grown in a pure culture, (3) the cultured microorganism should cause the disease when introduced into a healthy organism, and (4) the microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

**Why the Correct Answer is Right**
Koch's postulates are not always applicable in modern medicine, particularly with the discovery of viruses and other microorganisms that do not meet the criteria. For instance, viruses cannot be grown in a pure culture, and they often cause disease through mechanisms that do not involve direct invasion of host cells. However, one organism that does not satisfy Koch's postulate is **Mycoplasma pneumoniae**. This bacterium cannot be grown in a pure culture using traditional methods and does not meet the first postulate, as it is often found in healthy individuals as well as those with the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because, while some bacteria may not satisfy all of Koch's postulates, they are still subject to the criteria.
**Option B:** This option is incorrect because viruses are the primary example of organisms that do not meet Koch's postulates, not bacteria.
**Option C:** This option is incorrect because some bacteria, such as **Mycoplasma pneumoniae**, do not satisfy Koch's postulates, but this is not unique to them.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Koch's postulates are not absolute and have been modified over time to accommodate new discoveries in microbiology. However, they remain a useful framework for understanding the relationship between microorganisms and disease.

**Correct Answer: D. Mycoplasma pneumoniae**