Also I would add that the phrase "is to be taken" or "is to" or "are to" is pretty formal. You wouldn't hear two people casually talking using that phrase. I would think of it more as probably part of a larger set of rules or guidelines set forth by some authority, applied to a specific situation, where they are written down maybe in a handbook or posted on a sign somewhere. Does that make sense? So yes what the above poster says would apply - if you're giving those directions to someone as a suggestion, use "should not", and if you're giving it as a command that is to be obeyed then use "is not to be" or "is to be" or whatever. Hope that helps

Yes, you're right. You can think of "to be + past participle" to be similar to "should be + past participle" in the context you have noted. However, "should" is a modal verb that implies an obligation/duty, whereas "to be + past participle" is stronger and acts as an imperative in this specific context. In other words, think of "should be + past participle" as mere advice, and "to be + past participle" as a command.

No food is to be taken is a bit stronger that if it were "should be". No one shoul exceed the posted speed limit when driving but many people do drive faster. People obey rules when they see the benefit of compliance. Monetary fines are meaningless to some people. They are a cost of doing business. If the understand that their loved one could be harmed if they contaminate a sickroom they may obey the rule.