Filter exit subject to contamination?

[TheToe]

I have an (Omni) under-the-sink water filter which has been in place for ~7 years. The line from the filter to the faucet is a plastic tube which is noticeably discolored: a grayish brown.

I wrote to the manufacturer and they said the discoloration is entirely from carbon fines which got dislodged from the filter.

My wife thinks it is from bacteria growing in the line. Her reasoning: once the water is filtered, there is no significant chlorine in the exit line. Further, when I replace the filters, air gets into the tube. Bacteria travels on airborne dust, so it is possible that it could be getting into the line.

Does anyone know generally if there is concern about contamination of filter exit lines because they are not chlorinated? I understand that the filtered water itself is not contaminated, but again, air in the line can introduce foreign substances. It only takes one microscopic bacterium to start a major colony.