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Open flesh wounds that have been exposed to lake water are at risk for bacterial infection, especially when soft-tissue samples show fishy white blood cell counts. But when the cultures reveal nada, how is it possible that the inflamed tissue, with no organism invaders present, could grow a nice, big, green algae colony?

Two young men, involved in separate freshwater spills, contracted infections from a rare culprit, , of the University of Iowa, and colleagues, reported in the .

The first man, 24, sustained a deep puncture wound in his right foot in a Texas lake, and the second man, 30, dislocated his right knee and suffered a coronal unicondylar fracture in his distal right femur after wiping out on a jet ski at a lake in Missouri.

Surgically removed tissue samples from both men were tested, revealing polymorphonuclear leukocytes, but no organisms.

According to the authors, routine cultures, bacterial and mycobacterial, didn't raise any red flags. Only a single colony of coagulase-negative staphylococcus species was found in the tissue sample from the second man. Fungal cultures from tissue samples taken from both men grew bright green colonies, which were later identified as Desmodesmus armatus -- a chlorophyll-containing alga.

"It's fascinating that algal infections almost never occur even though algae are all around and we're frequently exposed to them," Ford said in an email to MedPage Today. "We were badly confused when we saw abundant grass-green colonies in our cultures as no fungus or bacteria anyone knew of looked anything like it."

"When an organism grows abundantly, in the absence of other organisms that cause infection, it suggests that that organism observed is the cause of the patient's infection, yet we had no idea what we had grown," he said.

"Algae in the genus Prototheca are known to cause human disease and those organisms have been relatively well characterized. However, Prototheca do not have chlorophyll and [to the] eye look something like ordinary colorless yeast when grown in culture."

Ford said that microscopically, the colony looked like nothing they'd ever seen before, but it did resemble algae. "It was around that time that Fader and Ranganath sent an email to ClinMicroNet, an email list populated by microbiology laboratory directors, relating that they were in a similar situation with an organism that was similar to ours in all respects, from the patient's characteristics to the appearance of the organism in culture."

Chlorophyll-containing algae have been known, anecdotally, to cause a disease called "chlorellosis" in humans and animals for decades because in pathological tissue sections, Ford said, "the causative organism looked like algae of the genus Chlorella." But, Ford's team identified the two samples as D. armatus, not Chlorella. "[We] find it interesting that both isolates are the same species according to our analysis."

Ford said what they all found so striking was the lack of another more typical cause of infection, the abundant growth, the unique bright green color of the colony, and that infections happened two times in separate areas of the country, with an algae of the same species.

Both men underwent surgical debridement without antifungal treatments. The man with the deep tissue puncture had no further problems, but the jet ski guy had recurrent methicillin-susceptible Staphylococcus aureus-associated septic arthritis.

The authors note two other cases of infection from chlorophyllic algae, one contracted from another puncture wound in the foot in a river, and the other from a gangrenous foot belonging to a patient with diabetes.

"We hope that our experience with treating these two cases, and the fact that we have demonstrated how to grow and identify an organism that causes this disease, will help physicians provide better care for patients who have this rare infection," Ford said.

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